fix(backend/api): Return empty array instead of 404 when no story segments exist

- GetSegmentsByLevelAsync now returns Ok([]) instead of NotFound() when no segments exist
- Prevents frontend from throwing errors when database has no story segments
- More RESTful: 200 with empty array vs 404 for non-existence

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

GermanApp/Presentation/Controllers/StoryController.cs

@@ -100,8 +100,8 @@ public class StoryController : ControllerBase
         
         if (!segments.Any())
         {
-            _logger.LogWarning("No story segments found for level {LevelId}", levelId);
+            _logger.LogInformation("No story segments found for level {LevelId}", levelId);
-            return NotFound();
+            return Ok(new List<StorySegmentDto>()); // Return empty list instead of NotFound
         }
 
         return Ok(segments);