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

tests/unit/test_nfo_generator.py

@@ -328,3 +328,78 @@ class TestNFOGeneratorEdgeCases:
         xml_string = generate_tvshow_nfo(nfo)
         
         assert "<premiered>2020-01-01</premiered>" in xml_string
+
+
+class TestFSKRatingGeneration:
+    """Test FSK rating generation in NFO XML."""
+    
+    def test_generate_nfo_with_fsk_rating(self):
+        """Test NFO generation with FSK rating."""
+        nfo = TVShowNFO(
+            title="FSK Show",
+            plot="Test",
+            fsk="FSK 12",
+            mpaa="TV-14"
+        )
+        
+        xml_string = generate_tvshow_nfo(nfo)
+        
+        # Should use FSK rating when available and preferred (default)
+        assert "<mpaa>FSK 12</mpaa>" in xml_string
+    
+    def test_generate_nfo_fsk_preferred_over_mpaa(self):
+        """Test that FSK is preferred over MPAA when both present."""
+        nfo = TVShowNFO(
+            title="FSK Priority Show",
+            plot="Test",
+            fsk="FSK 16",
+            mpaa="TV-MA"
+        )
+        
+        xml_string = generate_tvshow_nfo(nfo)
+        
+        # FSK should be in mpaa tag, not TV-MA
+        assert "<mpaa>FSK 16</mpaa>" in xml_string
+        assert "TV-MA" not in xml_string
+    
+    def test_generate_nfo_fallback_to_mpaa(self):
+        """Test fallback to MPAA when FSK not available."""
+        nfo = TVShowNFO(
+            title="MPAA Show",
+            plot="Test",
+            fsk=None,
+            mpaa="TV-PG"
+        )
+        
+        xml_string = generate_tvshow_nfo(nfo)
+        
+        # Should use MPAA when FSK not available
+        assert "<mpaa>TV-PG</mpaa>" in xml_string
+    
+    def test_generate_nfo_with_all_fsk_values(self):
+        """Test NFO generation with all possible FSK values."""
+        fsk_values = ["FSK 0", "FSK 6", "FSK 12", "FSK 16", "FSK 18"]
+        
+        for fsk in fsk_values:
+            nfo = TVShowNFO(
+                title=f"FSK {fsk} Show",
+                plot="Test",
+                fsk=fsk
+            )
+            
+            xml_string = generate_tvshow_nfo(nfo)
+            assert f"<mpaa>{fsk}</mpaa>" in xml_string
+    
+    def test_generate_nfo_no_rating(self):
+        """Test NFO generation when neither FSK nor MPAA is available."""
+        nfo = TVShowNFO(
+            title="No Rating Show",
+            plot="Test",
+            fsk=None,
+            mpaa=None
+        )
+        
+        xml_string = generate_tvshow_nfo(nfo)
+        
+        # mpaa tag should not be present
+        assert "<mpaa>" not in xml_string