Task 1.4: Update provider classes to use key as primary identifier

- Enhanced download() method docstring in aniworld_provider.py
- Enhanced Download() method docstring in enhanced_provider.py
- Clarified that 'key' is the series unique identifier from provider
- Clarified that 'serie_folder'/'serieFolder' is filesystem folder name (metadata only)
- Added comprehensive Args, Returns, and Raises sections to docstrings
- Fixed PEP 8 line length issue in logging statement
- Verified existing code already uses 'key' for identification and logging
- All 34 provider-related tests pass successfully
- No functional changes required, documentation improvements only

src/core/providers/enhanced_provider.py

@@ -349,7 +349,27 @@ class EnhancedAniWorldLoader(Loader):
         language: str = "German Dub",
         progress_callback: Optional[Callable] = None,
     ) -> bool:
-        """Download episode with comprehensive error handling."""
+        """Download episode with comprehensive error handling.
+        
+        Args:
+            baseDirectory: Base download directory path
+            serieFolder: Filesystem folder name (metadata only, used for
+                file path construction)
+            season: Season number (0 for movies)
+            episode: Episode number
+            key: Series unique identifier from provider (used for
+                identification and API calls)
+            language: Audio language preference (default: German Dub)
+            progress_callback: Optional callback for download progress
+                updates
+            
+        Returns:
+            bool: True if download succeeded, False otherwise
+            
+        Raises:
+            DownloadError: If download fails after all retry attempts
+            ValueError: If required parameters are missing or invalid
+        """
         self.download_stats["total_downloads"] += 1
 
         try: