feat: Implement NFOService.update_tvshow_nfo()

- Parse existing NFO to extract TMDB ID from uniqueid or tmdbid element
- Fetch fresh metadata from TMDB API
- Regenerate NFO with updated data
- Optionally re-download media files
- Add comprehensive error handling (missing NFO, no TMDB ID, invalid XML)
- Add unit tests for XML parsing logic (4 tests, all passing)
- Add integration test script (requires TMDB API key)

docs/task3_status.md

@@ -258,7 +258,7 @@ Task 3 is **95% Complete** and **Production Ready**.
 **⚠️ Documentation Remaining (5%):**
 
 -   📝 TMDB API setup guide (10 min)
--   📝 Configuration examples for README (10 min)  
+-   📝 Configuration examples for README (10 min)
 -   📝 ARCHITECTURE.md component diagram (10 min)
 
 **Optional Future Work (Not blocking):**