Fix Code Duplication 4: Create media utilities module

- Created src/server/utils/media.py with reusable media file functions
- Functions: check_media_files(), get_media_file_paths(), has_all_images(), count_video_files(), has_video_files()
- Defined standard filename constants: POSTER_FILENAME, LOGO_FILENAME, FANART_FILENAME, NFO_FILENAME
- Defined VIDEO_EXTENSIONS set for media player compatibility
- Refactored src/server/api/nfo.py (7 locations) to use utility functions
- Refactored src/server/services/background_loader_service.py to use utility
- Functions accept both str and Path for compatibility
- Marked Code Duplications 1, 3, 4 as RESOLVED in instructions.md
- Updated Further Considerations as RESOLVED (addressed in Issues 7, 9, 10)

docs/ARCHITECTURE.md

@@ -483,7 +483,7 @@ The application uses two complementary error handling mechanisms:
 # Base exception with HTTP status mapping
 AniWorldAPIException(message, status_code, error_code, details)
 ├── AuthenticationError (401)
-├── AuthorizationError (403)  
+├── AuthorizationError (403)
 ├── ValidationError (422)
 ├── NotFoundError (404)
 ├── ConflictError (409)
@@ -499,11 +499,13 @@ AniWorldAPIException(message, status_code, error_code, details)
 #### When to Use Each
 
 **Use HTTPException for:**
+
 - Simple parameter validation (missing fields, wrong type)
 - Direct HTTP-level errors (401, 403, 404 without business context)
 - Quick endpoint-specific failures
 
 **Use Custom Exceptions for:**
+
 - Service-layer business logic errors (AnimeServiceError, ConfigServiceError)
 - Errors needing rich context (details dict, error codes)
 - Errors that should be logged with specific categorization
@@ -530,6 +532,7 @@ except AnimeServiceError as e:
 #### Global Exception Handlers
 
 All custom exceptions are automatically handled by global middleware that:
+
 - Converts exceptions to structured JSON responses
 - Logs errors with appropriate severity
 - Includes request ID for tracking