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,

src/server/utils/dependencies.py

@@ -57,6 +57,44 @@ _rate_limit_lock = Lock()
 _RATE_LIMIT_WINDOW_SECONDS = 60.0
 
 
+def _make_db_lookup():
+    """Build a synchronous ``(folder) -> Serie | None`` callable for SerieScanner.
+
+    The returned function opens a short-lived sync DB session, queries for a
+    series whose ``folder`` column matches the given name, and converts the
+    ORM row to a ``Serie`` domain object.  Returns ``None`` when the DB is not
+    yet initialised or no matching row is found.
+    """
+    from src.core.entities.series import Serie
+
+    def _lookup(folder: str) -> Optional["Serie"]:
+        try:
+            from src.server.database.connection import get_sync_session
+            from src.server.database.service import AnimeSeriesService
+
+            db = get_sync_session()
+            try:
+                row = AnimeSeriesService.get_by_folder_sync(db, folder)
+            finally:
+                db.close()
+
+            if row is None:
+                return None
+            return Serie(
+                key=row.key,
+                name=row.name or "",
+                site=row.site,
+                folder=row.folder,
+                episodeDict={},
+                year=row.year,
+            )
+        except RuntimeError:
+            # DB not initialised yet (e.g. first boot before init_db())
+            return None
+
+    return _lookup
+
+
 def get_series_app() -> SeriesApp:
     """
     Dependency to get SeriesApp instance.
@@ -134,7 +172,7 @@ def get_series_app() -> SeriesApp:
                         ),
                     )
 
-            _series_app = SeriesApp(anime_dir)
+            _series_app = SeriesApp(anime_dir, db_lookup=_make_db_lookup())
         except Exception as e:
             raise HTTPException(
                 status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,