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/config/settings.py

@@ -109,6 +109,11 @@ class Settings(BaseSettings):
         validation_alias="NFO_IMAGE_SIZE",
         description="Image size to download (original, w500, etc.)"
     )
+    nfo_prefer_fsk_rating: bool = Field(
+        default=True,
+        validation_alias="NFO_PREFER_FSK_RATING",
+        description="Prefer German FSK rating over MPAA rating in NFO files"
+    )
 
     @property
     def allowed_origins(self) -> list[str]: