Add country metadata extraction and assignment features

- Updated `README.md` to include instructions for setting up the TMDB API key and new admin endpoints for managing country metadata.
- Implemented `/admin/missing-countries` endpoint to list media items without country metadata, with filtering options for source and media type.
- Added `/admin/assign-country` endpoint to manually assign a country code to a media item.
- Enhanced country extraction logic in `sync.py` to utilize TMDB and MusicBrainz APIs for automatic country retrieval based on available metadata.
- Updated configuration in `config.py` to include optional TMDB API key setting.
- Improved error handling and logging for country extraction failures.
- Ensured that country data is stored and utilized during media item synchronization across Radarr, Sonarr, and Lidarr.

backend/app/core/config.py

@@ -29,6 +29,9 @@ class Settings(BaseSettings):
     # Admin
     admin_token: Optional[str] = os.getenv("MOVIEMAP_ADMIN_TOKEN")
     
+    # External APIs (optional)
+    tmdb_api_key: str = os.getenv("TMDB_API_KEY", "")
+    
     @property
     def database_url(self) -> str:
         """Build PostgreSQL connection string using Unix socket"""