Add German FSK rating support for NFO files

- Add optional fsk field to TVShowNFO model
- Implement TMDB content ratings API integration
- Add FSK extraction and mapping (FSK 0/6/12/16/18)
- Update XML generation to prefer FSK over MPAA
- Add nfo_prefer_fsk_rating config setting
- Add 31 comprehensive tests for FSK functionality
- All 112 NFO tests passing

src/core/services/tmdb_client.py

@@ -191,6 +191,17 @@ class TMDBClient:
         
         return await self._request(f"tv/{tv_id}", params)
     
+    async def get_tv_show_content_ratings(self, tv_id: int) -> Dict[str, Any]:
+        """Get content ratings for a TV show.
+        
+        Args:
+            tv_id: TMDB TV show ID
+            
+        Returns:
+            Content ratings by country
+        """
+        return await self._request(f"tv/{tv_id}/content_ratings")
+    
     async def get_tv_show_external_ids(self, tv_id: int) -> Dict[str, Any]:
         """Get external IDs (IMDB, TVDB) for a TV show.