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/server/database/service.py

@@ -148,7 +148,27 @@ class AnimeSeriesService:
             select(AnimeSeries).where(AnimeSeries.key == key)
         )
         return result.scalar_one_or_none()
-    
+
+    @staticmethod
+    def get_by_folder_sync(db: Session, folder: str) -> Optional[AnimeSeries]:
+        """Look up an anime series by its filesystem folder name (sync).
+
+        Intended as a fallback for ``SerieScanner`` when neither a ``key``
+        file nor a ``data`` file exists on disk for a given folder.
+
+        Args:
+            db: Synchronous database session (from ``get_sync_session``).
+            folder: Filesystem folder name to match (e.g.
+                ``"Rooster Fighter (2026)"``).
+
+        Returns:
+            ``AnimeSeries`` instance or ``None`` if not found.
+        """
+        result = db.execute(
+            select(AnimeSeries).where(AnimeSeries.folder == folder)
+        )
+        return result.scalar_one_or_none()
+
     @staticmethod
     async def get_all(
         db: AsyncSession,