Add year extraction from folder names for existing series

- New migration script: populate year from folder (YYYY) pattern
- SerieScanner: refactor year extraction logic
- anime_service: pass year when syncing from data files

src/server/services/anime_service.py

@@ -1550,6 +1550,7 @@ async def sync_series_from_data_files(
                         name=serie.name,
                         site=serie.site,
                         folder=serie.folder,
+                        year=serie.year if hasattr(serie, 'year') else None,
                     )
                     
                     # Create Episode records for each episode in episodeDict