feat(scanner): add DB fallback for series key resolution

When SerieScanner encounters a folder without a local key or data file,
it now optionally falls back to a database lookup by folder name. This
prevents newly-added series from being silently skipped on rescan when
their metadata only lives in the DB.

Changes:
- SerieScanner accepts an optional db_lookup callable
- SeriesApp forwards db_lookup to SerieScanner
- AnimeSeriesService adds get_by_folder_sync() helper
- dependencies.py wires a sync DB lookup into get_series_app()
- Unit tests cover fallback hit, miss, and exception paths

src/core/SeriesApp.py

@@ -14,7 +14,7 @@ import asyncio
 import logging
 import os
 from concurrent.futures import ThreadPoolExecutor
-from typing import Any, Dict, List, Optional
+from typing import Any, Callable, Dict, List, Optional
 
 from events import Events
 
@@ -143,12 +143,16 @@ class SeriesApp:
     def __init__(
         self,
         directory_to_search: str,
+        db_lookup: Optional[Callable[[str], Optional["Serie"]]] = None,
     ):
         """
         Initialize SeriesApp.
 
         Args:
             directory_to_search: Base directory for anime series
+            db_lookup: Optional callable ``(folder_name) -> Serie | None``
+                passed through to ``SerieScanner`` as a fallback key source
+                when no local ``key`` or ``data`` file exists.
         """
 
         self.directory_to_search = directory_to_search
@@ -162,7 +166,7 @@ class SeriesApp:
         self.loaders = Loaders()
         self.loader = self.loaders.GetLoader(key="aniworld.to")
         self.serie_scanner = SerieScanner(
-            directory_to_search, self.loader
+            directory_to_search, self.loader, db_lookup=db_lookup
         )
         # Skip automatic loading from data files - series will be loaded
         # from database by the service layer during application setup