refactor: move sync_series_from_data_files to anime_service

- Moved _sync_series_to_database from fastapi_app.py to anime_service.py
- Renamed to sync_series_from_data_files for better clarity
- Updated all imports and test references
- Removed completed TODO tasks from instructions.md

src/server/services/anime_service.py

@@ -361,3 +361,84 @@ class AnimeService:
 def get_anime_service(series_app: SeriesApp) -> AnimeService:
     """Factory used for creating AnimeService with a SeriesApp instance."""
     return AnimeService(series_app)
+
+
+async def sync_series_from_data_files(
+    anime_directory: str,
+    logger=None
+) -> int:
+    """
+    Sync series from data files to the database.
+
+    Scans the anime directory for data files and adds any new series
+    to the database. Existing series are skipped (no duplicates).
+
+    This function is typically called during application startup to ensure
+    series metadata stored in filesystem data files is available in the
+    database.
+
+    Args:
+        anime_directory: Path to the anime directory with data files
+        logger: Optional logger instance for logging operations.
+            If not provided, uses structlog.
+
+    Returns:
+        Number of new series added to the database
+    """
+    log = logger or structlog.get_logger(__name__)
+
+    try:
+        from src.core.entities.SerieList import SerieList
+        from src.server.database.connection import get_db_session
+
+        log.info(
+            "Starting data file to database sync",
+            directory=anime_directory
+        )
+
+        # Get all series from data files using SeriesApp
+        series_app = SeriesApp(anime_directory)
+        all_series = await asyncio.to_thread(
+            series_app.get_all_series_from_data_files
+        )
+
+        if not all_series:
+            log.info("No series found in data files to sync")
+            return 0
+
+        log.info(
+            "Found series in data files, syncing to database",
+            count=len(all_series)
+        )
+
+        async with get_db_session() as db:
+            serie_list = SerieList(
+                anime_directory,
+                db_session=db,
+                skip_load=True
+            )
+            added_count = 0
+            for serie in all_series:
+                result = await serie_list.add_to_db(serie, db)
+                if result:
+                    added_count += 1
+                    log.debug(
+                        "Added series to database",
+                        name=serie.name,
+                        key=serie.key
+                    )
+
+            log.info(
+                "Data file sync complete",
+                added=added_count,
+                skipped=len(all_series) - added_count
+            )
+            return added_count
+
+    except Exception as e:
+        log.warning(
+            "Failed to sync series to database",
+            error=str(e),
+            exc_info=True
+        )
+        return 0