feat(backend/story-integration): Phase 4 - AI Integration with CEFR-level prompts

- Enhanced StoryGenerationService with CEFR-level-specific prompt templates
- Added GetStoryRequirements() method with level-specific requirements for A1-C1
- Added GetSegmentRequirements() method with level-specific requirements
- Story prompts now include detailed CEFR-level constraints:
  - A1: Simple present tense, basic SVO structure, max 500 words
  - A2: Present/past/future, subordinate clauses, max 1000 words
  - B1: All tenses, modal verbs, complex sentences, max 2000 words
  - B2: Nuanced tenses, all cases, Konjunktiv I/II, Passiv, max 3000 words
  - C1: All tenses/moods, sophisticated vocabulary, literary devices
- Updated docs/features/story-integration.md for Phase 4 completion
- All 324 tests still pass

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

GermanApp/Application/Services/StoryGenerationService.cs

@@ -344,21 +344,58 @@ public class StoryGenerationService
     private string BuildStoryPrompt(int levelId, string theme, IReadOnlyList<string> vocabulary, int segmentCount)
     {
         var levelCode = GetLevelCode(levelId);
-        var vocabularyString = string.Join(", ", vocabulary.Take(20)); // Limit to first 20 words
+        var vocabularyString = string.Join(", ", vocabulary.Take(20));
         
         if (vocabulary.Count > 20)
         {
             vocabularyString += ", ...";
         }
 
+        var requirements = GetStoryRequirements(levelCode);
         return $@"Write a {segmentCount}-part continuous story for a German {levelCode} learner. 
 The story theme is: {theme}. 
 Include these German words and phrases: {vocabularyString}. 
-Use only {levelCode} level vocabulary and grammar. 
-Write each part as a separate paragraph. 
-Make the story engaging and appropriate for language learners.";
+
+REQUIREMENTS:
+{requirements}";
     }
 
+    private string GetStoryRequirements(string levelCode) => levelCode switch
+    {
+        "A1" => "- Use ONLY A1-level vocabulary and grammar\n" +
+               "- Use simple present tense (ich bin, ich habe, ich gehe)\n" +
+               "- Basic sentence structure: Subject-Verb-Object\n" +
+               "- Vocabulary: max 500 words, sentence length: max 10 words\n" +
+               "- Each part: 2-3 short sentences\n" +
+               "- Do NOT use: subjunctive, genitive, complex prepositions",
+        "A2" => "- Use ONLY A2-level vocabulary and grammar\n" +
+               "- Use present, past (Perfekt), future tense\n" +
+               "- Can use subordinate clauses with weil, dass, wenn\n" +
+               "- Vocabulary: 500-1000 words, sentence length: max 15 words\n" +
+               "- Each part: 3-4 sentences\n" +
+               "- Do NOT use: Konjunktiv II, Passiv, complex noun compounds",
+        "B1" => "- Use ONLY B1-level vocabulary and grammar\n" +
+               "- Use all tenses: present, past (Praeteritum/Perfekt), future\n" +
+               "- Use modal verbs: koennen, muessen, duerfen, sollen, wollen, moegen\n" +
+               "- Sentence structure: complex sentences with subordinate clauses\n" +
+               "- Vocabulary: 1000-2000 words, sentence length: max 20 words\n" +
+               "- Each part: 4-5 sentences",
+        "B2" => "- Use ONLY B2-level vocabulary and grammar\n" +
+               "- Use nuanced tenses: Present, Praeteritum, Perfekt, Plusquamperfekt, Future I/II\n" +
+               "- Use all cases: Nominativ, Akkusativ, Dativ, Genitiv\n" +
+               "- Use Konjunktiv I (indirect speech) and Konjunktiv II\n" +
+               "- Use Passiv where appropriate\n" +
+               "- Vocabulary: 2000-3000 words, sentence length: max 25 words\n" +
+               "- Each part: 5-6 sentences",
+        "C1" => "- Use C1-level vocabulary and grammar with precision\n" +
+               "- Use all tenses and moods: Indikativ, Konjunktiv I/II, Passiv in all forms\n" +
+               "- Sentence structure: highly complex with nested relative clauses\n" +
+               "- Use sophisticated vocabulary including idiomatic expressions\n" +
+               "- Vocabulary: 3000-5000 words, sentence length: can exceed 25 words\n" +
+               "- Each part: 6-8 sentences",
+        _ => "- Use only " + levelCode + " level vocabulary and grammar"
+    };
+
     /// <summary>
     /// Builds a prompt for a single story segment.
     /// </summary>
@@ -375,13 +412,44 @@ Make the story engaging and appropriate for language learners.";
             vocabularyString += ", ...";
         }
 
-        return $@"Write a short story segment (5-8 sentences) for a German {levelCode} learner. 
+        var sentenceCount = levelCode switch { "A1" => "3-5", "A2" => "4-6", "B1" => "5-7", "B2" => "6-8", "C1" => "7-10", _ => "5-8" };
+        var requirements = GetSegmentRequirements(levelCode);
+        return $@"Write a short story segment ({sentenceCount} sentences) for a German {levelCode} learner. 
 The theme is: {theme}. 
 Include these German words: {vocabularyString}. 
-Use only {levelCode} level vocabulary and grammar. 
-Make it engaging and appropriate for language learners.";
+
+REQUIREMENTS:
+{requirements}";
     }
 
+    private string GetSegmentRequirements(string levelCode) => levelCode switch
+    {
+        "A1" => "- Use ONLY A1-level vocabulary and grammar\n" +
+               "- Use simple present tense only (ich bin, ich habe, ich gehe)\n" +
+               "- Basic sentence structure: Subject-Verb-Object\n" +
+               "- Sentence length: max 8 words\n" +
+               "- Do NOT use: subjunctive, genitive, complex prepositions",
+        "A2" => "- Use ONLY A2-level vocabulary and grammar\n" +
+               "- Use present, past (Perfekt), future tense\n" +
+               "- Can use simple subordinate clauses with weil, dass, wenn\n" +
+               "- Sentence length: max 12 words",
+        "B1" => "- Use ONLY B1-level vocabulary and grammar\n" +
+               "- Use present, past (Praeteritum/Perfekt), future tense\n" +
+               "- Use modal verbs: koennen, muessen, duerfen, sollen, wollen\n" +
+               "- Sentence structure: complex sentences with subordinate clauses\n" +
+               "- Sentence length: max 20 words",
+        "B2" => "- Use ONLY B2-level vocabulary and grammar\n" +
+               "- Use nuanced tenses including Plusquamperfekt and Konjunktiv II\n" +
+               "- Use all cases: Nominativ, Akkusativ, Dativ, Genitiv\n" +
+               "- Use Passiv where appropriate\n" +
+               "- Sentence length: max 25 words",
+        "C1" => "- Use C1-level vocabulary and grammar with precision\n" +
+               "- Use all tenses, moods, and cases correctly\n" +
+               "- Use sophisticated vocabulary including idiomatic expressions\n" +
+               "- Sentence structure: complex nested sentences with multiple clauses",
+        _ => "- Use only " + levelCode + " level vocabulary and grammar"
+    };
+
     /// <summary>
     /// Splits a full story text into segments.
     /// </summary>

docs/features/story-integration.md

@@ -1,7 +1,7 @@
 # Feature: Story Integration
 
 > **Status**: 🚀 In Progress  
-> **📊 Current Progress**: Phase 1-3 ✅ Complete (Database & Models, Backend Services, Unit Tests), Phase 4 Next (AI Integration)
+> **📊 Current Progress**: Phase 1-4 ✅ Complete (Database & Models, Backend Services, Unit Tests, AI Integration), Phase 5 Next (Audio Generation)
 > **Priority**: High  
 > **Complexity**: High  
 > **Estimate**: 8-12 hours  
@@ -157,13 +157,13 @@ Order: 1
 - [x] Write unit tests for StoryProgressRepository (Tests/Unit/Infrastructure/Data/Repositories/StoryProgressRepositoryTests.cs)
 - [x] All 324 tests pass
 
-### Phase 4: AI Integration (2-3 hours)
-- [ ] Configure Mistral-Medium API client
-- [ ] Create prompt templates for each level
-- [ ] Implement vocabulary extraction from lessons
-- [ ] Implement story text segmentation
-- [ ] Handle AI API errors gracefully
-- [ ] Add retry logic for failed generations
+### Phase 4: AI Integration (2-3 hours) - COMPLETE
+- [x] Configure Mistral-Medium API client (MistralConfig with retry, rate limiting, circuit breaker)
+- [x] Create prompt templates for each level (A1-C1 with CEFR-specific requirements)
+- [x] Implement vocabulary extraction from lessons (ExtractVocabularyFromLessons)
+- [x] Implement story text segmentation (SplitStoryIntoSegments, AdjustSegmentCount)
+- [x] Handle AI API errors gracefully (AiServiceException with error codes)
+- [x] Add retry logic for failed generations (ExecuteWithRetryAsync with exponential backoff)
 
 ### Phase 5: Audio Generation (2 hours)
 - [ ] Integrate with Coqui TTS service
@@ -193,7 +193,7 @@ Order: 1
 | Database & Models | June 13, 2025 | ✅ |
 | Backend Services | June 13, 2025 | ✅ |
 | Unit Tests | June 13, 2025 | ✅ |
-| AI Integration | - | ⏳ |
+| AI Integration | June 13, 2025 | ✅ |
 | Audio Generation | - | ⏳ |
 | Frontend Integration | - | ⏳ |
 | User Progress | - | ⏳ |
@@ -416,6 +416,7 @@ Make the story engaging and suitable for adult learners.
 | May 31, 2025 | Created | Initial plan based on application-plan.md |
 | June 13, 2025 | Phase 1-2 Complete | Database & Models, Backend Services implemented |
 | June 13, 2025 | Phase 3 Complete | All unit tests written and passing (324 tests) |
+| June 13, 2025 | Phase 4 Complete | Enhanced prompts with CEFR-level-specific requirements for story generation |
 
 ---