feat(backend/application): add DTOs and services for Lesson Management

- Created LevelDto.cs with LevelDto, CreateLevelDto, UpdateLevelDto
- Created UserProgressDto.cs with UserProgressDto, UpdateUserProgressDto, LevelCompletionDto
- Created LevelService.cs with CRUD operations for CEFR levels
- Created LessonService.cs with CRUD operations and filtering by level
- Created ProgressService.cs with user progress tracking, completion percentage, and lesson unlocking logic
- Registered all new services in Program.cs
- Fixed UserProgressDto ToEntity/UpdateFromDto to use domain entity methods

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

GermanApp/Application/DTOs/LevelDto.cs

@@ -0,0 +1,51 @@
+using GermanApp.Domain.Entities;
+
+namespace GermanApp.Application.DTOs;
+
+/// <summary>
+/// Data Transfer Object for Level - used for API responses.
+/// This is a read-only representation of a Level entity.
+/// </summary>
+public record LevelDto(
+    int Id,
+    string Name,
+    string Code,
+    int Order);
+
+/// <summary>
+/// Data Transfer Object for creating a new Level.
+/// </summary>
+public record CreateLevelDto(
+    string Name,
+    string Code,
+    int Order);
+
+/// <summary>
+/// Data Transfer Object for updating an existing Level.
+/// </summary>
+public record UpdateLevelDto(
+    string Name,
+    string Code,
+    int Order);
+
+/// <summary>
+/// Extension methods for mapping between Level entity and DTOs.
+/// </summary>
+public static class LevelDtoExtensions
+{
+    public static LevelDto ToDto(this Level level) => new(
+        level.Id,
+        level.Name,
+        level.Code,
+        level.Order);
+
+    public static Level ToEntity(this CreateLevelDto dto) =>
+        Level.Create(dto.Name, dto.Code, dto.Order);
+
+    public static void UpdateFromDto(this Level level, UpdateLevelDto dto)
+    {
+        level.UpdateName(dto.Name);
+        level.UpdateCode(dto.Code);
+        level.UpdateOrder(dto.Order);
+    }
+}

GermanApp/Application/DTOs/UserProgressDto.cs

@@ -0,0 +1,80 @@
+using GermanApp.Domain.Entities;
+
+namespace GermanApp.Application.DTOs;
+
+/// <summary>
+/// Data Transfer Object for UserProgress - used for API responses.
+/// This is a read-only representation of a UserProgress entity.
+/// </summary>
+public record UserProgressDto(
+    int Id,
+    int UserId,
+    int LessonId,
+    bool IsCompleted,
+    int QuizScore,
+    DateTime LastAttemptDate,
+    string? LessonTitle = null,
+    string? LevelName = null,
+    string? LevelCode = null);
+
+/// <summary>
+/// Data Transfer Object for creating/updating user progress.
+/// </summary>
+public record UpdateUserProgressDto(
+    int LessonId,
+    bool IsCompleted,
+    int QuizScore);
+
+/// <summary>
+/// Data Transfer Object for level completion summary.
+/// </summary>
+public record LevelCompletionDto(
+    int LevelId,
+    string LevelName,
+    string LevelCode,
+    int TotalLessons,
+    int CompletedLessons,
+    double CompletionPercentage);
+
+/// <summary>
+/// Extension methods for mapping between UserProgress entity and DTOs.
+/// </summary>
+public static class UserProgressDtoExtensions
+{
+    public static UserProgressDto ToDto(this UserProgress userProgress) => new(
+        userProgress.Id,
+        userProgress.UserId,
+        userProgress.LessonId,
+        userProgress.IsCompleted,
+        userProgress.QuizScore,
+        userProgress.LastAttemptDate,
+        userProgress.Lesson?.Title,
+        userProgress.Lesson?.Level?.Name,
+        userProgress.Lesson?.Level?.Code);
+
+    public static UserProgress ToEntity(this UpdateUserProgressDto dto, int userId)
+    {
+        var progress = UserProgress.Create(userId, dto.LessonId);
+        if (dto.IsCompleted)
+        {
+            progress.MarkAsCompleted(dto.QuizScore);
+        }
+        else
+        {
+            progress.UpdateQuizScore(dto.QuizScore);
+        }
+        return progress;
+    }
+
+    public static void UpdateFromDto(this UserProgress userProgress, UpdateUserProgressDto dto)
+    {
+        if (dto.IsCompleted)
+        {
+            userProgress.MarkAsCompleted(dto.QuizScore);
+        }
+        else
+        {
+            userProgress.UpdateQuizScore(dto.QuizScore);
+        }
+    }
+}

GermanApp/Application/Services/LessonService.cs

@@ -0,0 +1,159 @@
+using GermanApp.Application.DTOs;
+using GermanApp.Domain.Entities;
+using GermanApp.Domain.Interfaces;
+
+namespace GermanApp.Application.Services;
+
+/// <summary>
+/// Application service for managing lessons.
+/// This is part of the Application layer.
+/// </summary>
+public class LessonService
+{
+    private readonly ILessonRepository _lessonRepository;
+    private readonly ILevelRepository _levelRepository;
+
+    public LessonService(ILessonRepository lessonRepository, ILevelRepository levelRepository)
+    {
+        _lessonRepository = lessonRepository;
+        _levelRepository = levelRepository;
+    }
+
+    /// <summary>
+    /// Gets all lessons.
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetAllLessonsAsync(CancellationToken cancellationToken = default)
+    {
+        var lessons = await _lessonRepository.GetAllAsync(cancellationToken);
+        return lessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets all lessons ordered by level and lesson order.
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetAllOrderedLessonsAsync(CancellationToken cancellationToken = default)
+    {
+        var lessons = await _lessonRepository.GetAllOrderedAsync(cancellationToken);
+        return lessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets a lesson by its ID.
+    /// </summary>
+    public async Task<LessonDto?> GetLessonByIdAsync(int id, CancellationToken cancellationToken = default)
+    {
+        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
+        return lesson?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets lessons by level ID.
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetLessonsByLevelAsync(int levelId, CancellationToken cancellationToken = default)
+    {
+        var lessons = await _lessonRepository.GetByLevelAsync(levelId, cancellationToken);
+        return lessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets beginner lessons (A1 and A2 - levels 1 and 2).
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetBeginnerLessonsAsync(CancellationToken cancellationToken = default)
+    {
+        var level1Lessons = await _lessonRepository.GetByLevelAsync(1, cancellationToken);
+        var level2Lessons = await _lessonRepository.GetByLevelAsync(2, cancellationToken);
+        var allLessons = level1Lessons.Concat(level2Lessons)
+            .OrderBy(l => l.LevelId)
+            .ThenBy(l => l.Order)
+            .ToList();
+        return allLessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets advanced lessons (B2 and C1 - levels 4 and 5).
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetAdvancedLessonsAsync(CancellationToken cancellationToken = default)
+    {
+        var level4Lessons = await _lessonRepository.GetByLevelAsync(4, cancellationToken);
+        var level5Lessons = await _lessonRepository.GetByLevelAsync(5, cancellationToken);
+        var allLessons = level4Lessons.Concat(level5Lessons)
+            .OrderBy(l => l.LevelId)
+            .ThenBy(l => l.Order)
+            .ToList();
+        return allLessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Creates a new lesson.
+    /// </summary>
+    public async Task<LessonDto> CreateLessonAsync(CreateLessonDto dto, CancellationToken cancellationToken = default)
+    {
+        // Validate that the level exists
+        var level = await _levelRepository.GetByIdAsync(dto.LevelId, cancellationToken);
+        if (level == null)
+            throw new ArgumentException("Level does not exist");
+
+        var lesson = dto.ToEntity();
+        var createdLesson = await _lessonRepository.AddAsync(lesson, cancellationToken);
+        return createdLesson.ToDto();
+    }
+
+    /// <summary>
+    /// Updates an existing lesson.
+    /// </summary>
+    public async Task<LessonDto?> UpdateLessonAsync(int id, UpdateLessonDto dto, CancellationToken cancellationToken = default)
+    {
+        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
+        if (lesson == null)
+            return null;
+
+        // Validate that the level exists
+        var level = await _levelRepository.GetByIdAsync(dto.LevelId, cancellationToken);
+        if (level == null)
+            throw new ArgumentException("Level does not exist");
+
+        lesson.UpdateFromDto(dto);
+        await _lessonRepository.UpdateAsync(lesson, cancellationToken);
+        return lesson.ToDto();
+    }
+
+    /// <summary>
+    /// Deletes a lesson by its ID.
+    /// </summary>
+    public async Task<bool> DeleteLessonAsync(int id, CancellationToken cancellationToken = default)
+    {
+        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
+        if (lesson == null)
+            return false;
+
+        await _lessonRepository.DeleteAsync(lesson, cancellationToken);
+        return true;
+    }
+
+    /// <summary>
+    /// Gets the first lesson in a level.
+    /// </summary>
+    public async Task<LessonDto?> GetFirstLessonInLevelAsync(int levelId, CancellationToken cancellationToken = default)
+    {
+        var lesson = await _lessonRepository.GetFirstLessonInLevelAsync(levelId, cancellationToken);
+        return lesson?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets the next lesson after the specified one.
+    /// </summary>
+    public async Task<LessonDto?> GetNextLessonAsync(int currentLessonId, CancellationToken cancellationToken = default)
+    {
+        var lesson = await _lessonRepository.GetNextLessonAsync(currentLessonId, cancellationToken);
+        return lesson?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets lessons that the user can access (based on completion of previous lessons).
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetAccessibleLessonsAsync(int userId, CancellationToken cancellationToken = default)
+    {
+        var lessons = await _lessonRepository.GetAccessibleLessonsAsync(userId, cancellationToken);
+        return lessons.Select(l => l.ToDto()).ToList();
+    }
+}

GermanApp/Application/Services/LevelService.cs

@@ -0,0 +1,101 @@
+using GermanApp.Application.DTOs;
+using GermanApp.Domain.Entities;
+using GermanApp.Domain.Interfaces;
+
+namespace GermanApp.Application.Services;
+
+/// <summary>
+/// Application service for managing CEFR levels.
+/// This is part of the Application layer.
+/// </summary>
+public class LevelService
+{
+    private readonly ILevelRepository _levelRepository;
+
+    public LevelService(ILevelRepository levelRepository)
+    {
+        _levelRepository = levelRepository;
+    }
+
+    /// <summary>
+    /// Gets all CEFR levels ordered by their sort order.
+    /// </summary>
+    public async Task<IReadOnlyList<LevelDto>> GetAllLevelsAsync(CancellationToken cancellationToken = default)
+    {
+        var levels = await _levelRepository.GetAllOrderedAsync(cancellationToken);
+        return levels.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets a level by its ID.
+    /// </summary>
+    public async Task<LevelDto?> GetLevelByIdAsync(int id, CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetByIdAsync(id, cancellationToken);
+        return level?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets a level by its code (e.g., "A1", "B2").
+    /// </summary>
+    public async Task<LevelDto?> GetLevelByCodeAsync(string code, CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetByCodeAsync(code, cancellationToken);
+        return level?.ToDto();
+    }
+
+    /// <summary>
+    /// Creates a new CEFR level.
+    /// </summary>
+    public async Task<LevelDto> CreateLevelAsync(CreateLevelDto dto, CancellationToken cancellationToken = default)
+    {
+        var level = dto.ToEntity();
+        var createdLevel = await _levelRepository.AddAsync(level, cancellationToken);
+        return createdLevel.ToDto();
+    }
+
+    /// <summary>
+    /// Updates an existing CEFR level.
+    /// </summary>
+    public async Task<LevelDto?> UpdateLevelAsync(int id, UpdateLevelDto dto, CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetByIdAsync(id, cancellationToken);
+        if (level == null)
+            return null;
+
+        level.UpdateFromDto(dto);
+        await _levelRepository.UpdateAsync(level, cancellationToken);
+        return level.ToDto();
+    }
+
+    /// <summary>
+    /// Deletes a CEFR level by its ID.
+    /// </summary>
+    public async Task<bool> DeleteLevelAsync(int id, CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetByIdAsync(id, cancellationToken);
+        if (level == null)
+            return false;
+
+        await _levelRepository.DeleteAsync(level, cancellationToken);
+        return true;
+    }
+
+    /// <summary>
+    /// Gets the first level (lowest order number) - typically A1.
+    /// </summary>
+    public async Task<LevelDto?> GetFirstLevelAsync(CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetFirstLevelAsync(cancellationToken);
+        return level?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets the next level after the specified one.
+    /// </summary>
+    public async Task<LevelDto?> GetNextLevelAsync(int currentLevelId, CancellationToken cancellationToken = default)
+    {
+        var level = await _levelRepository.GetNextLevelAsync(currentLevelId, cancellationToken);
+        return level?.ToDto();
+    }
+}

GermanApp/Application/Services/ProgressService.cs

@@ -0,0 +1,206 @@
+using GermanApp.Application.DTOs;
+using GermanApp.Domain.Entities;
+using GermanApp.Domain.Interfaces;
+
+namespace GermanApp.Application.Services;
+
+/// <summary>
+/// Application service for managing user progress through lessons.
+/// This is part of the Application layer.
+/// </summary>
+public class ProgressService
+{
+    private readonly IUserProgressRepository _userProgressRepository;
+    private readonly ILessonRepository _lessonRepository;
+    private readonly ILevelRepository _levelRepository;
+
+    public ProgressService(
+        IUserProgressRepository userProgressRepository,
+        ILessonRepository lessonRepository,
+        ILevelRepository levelRepository)
+    {
+        _userProgressRepository = userProgressRepository;
+        _lessonRepository = lessonRepository;
+        _levelRepository = levelRepository;
+    }
+
+    /// <summary>
+    /// Gets user progress for a specific lesson.
+    /// </summary>
+    public async Task<UserProgressDto?> GetUserProgressAsync(int userId, int lessonId, CancellationToken cancellationToken = default)
+    {
+        var progress = await _userProgressRepository.GetByUserAndLessonAsync(userId, lessonId, cancellationToken);
+        return progress?.ToDto();
+    }
+
+    /// <summary>
+    /// Gets all progress records for a user.
+    /// </summary>
+    public async Task<IReadOnlyList<UserProgressDto>> GetUserProgressAsync(int userId, CancellationToken cancellationToken = default)
+    {
+        var progressRecords = await _userProgressRepository.GetByUserAsync(userId, cancellationToken);
+        return progressRecords.Select(p => p.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Gets progress for all lessons in a specific level for a user.
+    /// </summary>
+    public async Task<IReadOnlyList<UserProgressDto>> GetUserProgressByLevelAsync(int userId, int levelId, CancellationToken cancellationToken = default)
+    {
+        var progressRecords = await _userProgressRepository.GetByUserAndLevelAsync(userId, levelId, cancellationToken);
+        return progressRecords.Select(p => p.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Checks if a user has completed a lesson.
+    /// </summary>
+    public async Task<bool> HasUserCompletedLessonAsync(int userId, int lessonId, CancellationToken cancellationToken = default)
+    {
+        return await _userProgressRepository.HasUserCompletedLessonAsync(userId, lessonId, cancellationToken);
+    }
+
+    /// <summary>
+    /// Marks a lesson as completed for a user with a quiz score.
+    /// </summary>
+    public async Task<UserProgressDto> MarkLessonAsCompletedAsync(int userId, int lessonId, int quizScore, CancellationToken cancellationToken = default)
+    {
+        var progress = await _userProgressRepository.GetByUserAndLessonAsync(userId, lessonId, cancellationToken);
+        
+        if (progress == null)
+        {
+            // Create new progress record
+            progress = UserProgress.Create(userId, lessonId);
+            progress.MarkAsCompleted(quizScore);
+            await _userProgressRepository.AddAsync(progress, cancellationToken);
+        }
+        else
+        {
+            // Update existing progress record
+            progress.MarkAsCompleted(quizScore);
+            await _userProgressRepository.UpdateAsync(progress, cancellationToken);
+        }
+        
+        return progress.ToDto();
+    }
+
+    /// <summary>
+    /// Updates user progress for a lesson without marking as completed.
+    /// </summary>
+    public async Task<UserProgressDto> UpdateUserProgressAsync(int userId, UpdateUserProgressDto dto, CancellationToken cancellationToken = default)
+    {
+        var progress = await _userProgressRepository.GetByUserAndLessonAsync(userId, dto.LessonId, cancellationToken);
+        
+        if (progress == null)
+        {
+            // Create new progress record
+            progress = dto.ToEntity(userId);
+            await _userProgressRepository.AddAsync(progress, cancellationToken);
+        }
+        else
+        {
+            // Update existing progress record
+            progress.UpdateFromDto(dto);
+            await _userProgressRepository.UpdateAsync(progress, cancellationToken);
+        }
+        
+        return progress.ToDto();
+    }
+
+    /// <summary>
+    /// Resets user progress for a lesson.
+    /// </summary>
+    public async Task<bool> ResetLessonProgressAsync(int userId, int lessonId, CancellationToken cancellationToken = default)
+    {
+        var progress = await _userProgressRepository.GetByUserAndLessonAsync(userId, lessonId, cancellationToken);
+        if (progress == null)
+            return false;
+
+        progress.Reset();
+        await _userProgressRepository.UpdateAsync(progress, cancellationToken);
+        return true;
+    }
+
+    /// <summary>
+    /// Gets the user's average score for a level.
+    /// </summary>
+    public async Task<double> GetAverageScoreForLevelAsync(int userId, int levelId, CancellationToken cancellationToken = default)
+    {
+        return await _userProgressRepository.GetAverageScoreForLevelAsync(userId, levelId, cancellationToken);
+    }
+
+    /// <summary>
+    /// Gets the percentage of lessons completed in a level.
+    /// </summary>
+    public async Task<double> GetLevelCompletionPercentageAsync(int userId, int levelId, CancellationToken cancellationToken = default)
+    {
+        return await _userProgressRepository.GetLevelCompletionPercentageAsync(userId, levelId, cancellationToken);
+    }
+
+    /// <summary>
+    /// Gets level completion summaries for a user.
+    /// </summary>
+    public async Task<IReadOnlyList<LevelCompletionDto>> GetLevelCompletionsAsync(int userId, CancellationToken cancellationToken = default)
+    {
+        var levels = await _levelRepository.GetAllOrderedAsync(cancellationToken);
+        var completions = new List<LevelCompletionDto>();
+
+        foreach (var level in levels)
+        {
+            var lessons = await _lessonRepository.GetByLevelAsync(level.Id, cancellationToken);
+            var completedCount = 0;
+            
+            foreach (var lesson in lessons)
+            {
+                var hasCompleted = await _userProgressRepository.HasUserCompletedLessonAsync(userId, lesson.Id, cancellationToken);
+                if (hasCompleted)
+                    completedCount++;
+            }
+            
+            var percentage = lessons.Count > 0 
+                ? (double)completedCount / lessons.Count * 100 
+                : 0;
+            
+            completions.Add(new LevelCompletionDto(
+                level.Id,
+                level.Name,
+                level.Code,
+                lessons.Count,
+                completedCount,
+                percentage
+            ));
+        }
+        
+        return completions;
+    }
+
+    /// <summary>
+    /// Gets lessons that the user can access (completed previous lessons or first in level).
+    /// </summary>
+    public async Task<IReadOnlyList<LessonDto>> GetAccessibleLessonsAsync(int userId, CancellationToken cancellationToken = default)
+    {
+        var lessons = await _lessonRepository.GetAccessibleLessonsAsync(userId, cancellationToken);
+        return lessons.Select(l => l.ToDto()).ToList();
+    }
+
+    /// <summary>
+    /// Checks if the next lesson in a level is unlocked for the user.
+    /// </summary>
+    public async Task<bool> IsNextLessonUnlockedAsync(int userId, int currentLessonId, CancellationToken cancellationToken = default)
+    {
+        var currentLesson = await _lessonRepository.GetByIdAsync(currentLessonId, cancellationToken);
+        if (currentLesson == null)
+            return false;
+
+        // If this is the first lesson in the level, the next one is always unlocked
+        var firstLesson = await _lessonRepository.GetFirstLessonInLevelAsync(currentLesson.LevelId, cancellationToken);
+        if (firstLesson?.Id == currentLessonId)
+        {
+            var nextLesson = await _lessonRepository.GetNextLessonAsync(currentLessonId, cancellationToken);
+            return nextLesson != null;
+        }
+
+        // Check if the user has completed the current lesson
+        var hasCompleted = await _userProgressRepository.HasUserCompletedLessonAsync(userId, currentLessonId, cancellationToken);
+        return hasCompleted;
+    }
+}

GermanApp/Program.cs

@@ -1,4 +1,5 @@
 using GermanApp.Application.DTOs;
+using GermanApp.Application.Services;
 using GermanApp.Application.Interfaces;
 using GermanApp.Application.UseCases.Commands;
 using GermanApp.Domain.Entities;
@@ -115,13 +116,20 @@ try
     });
 
     // Register repositories (Infrastructure implementations of Domain interfaces)
+    builder.Services.AddScoped<ILevelRepository, LevelRepository>();
     builder.Services.AddScoped<ILessonRepository, LessonRepository>();
+    builder.Services.AddScoped<IUserProgressRepository, UserProgressRepository>();
 
     // ============================================
     // APPLICATION LAYER - Use Cases & Services
     // ============================================
 
     // Register command handlers
+
+    // Register application services
+    builder.Services.AddScoped<LevelService>();
+    builder.Services.AddScoped<LessonService>();
+    builder.Services.AddScoped<ProgressService>();
     builder.Services.AddScoped<ICommandHandler<CreateLessonCommand, LessonDto>, CreateLessonCommandHandler>();
 
     var app = builder.Build();