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/entities/nfo_models.py

@@ -175,6 +175,10 @@ class TVShowNFO(BaseModel):
         description="Episode runtime in minutes"
     )
     mpaa: Optional[str] = Field(None, description="Content rating")
+    fsk: Optional[str] = Field(
+        None,
+        description="German FSK rating (e.g., 'FSK 12', 'FSK 16')"
+    )
     certification: Optional[str] = Field(
         None,
         description="Certification info"