feat(SerieScanner): DB lookup primary, deprecate key file fallback

DB now source of truth for folder -> Serie resolution.

Changes:
- AnimeSeriesService.get_by_folder(): new async lookup by folder name
- SerieScanner.__read_data_from_file(): query DB first, then provider callback, then legacy key file (temporary, removed v3.0.0)
- Serie: reconstruct from DB record with episode dict
- Key file: warn on use, scheduled removal v3.0.0

Add unit tests for DB hit/miss/callback/fallback edge cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/database/service.py

@@ -169,6 +169,26 @@ class AnimeSeriesService:
         )
         return result.scalar_one_or_none()
 
+    @staticmethod
+    async def get_by_folder(db: AsyncSession, folder: str) -> Optional[AnimeSeries]:
+        """Look up an anime series by its filesystem folder name (async).
+
+        Intended as primary lookup for ``SerieScanner`` when scanning
+        directories, replacing the legacy file-based lookups (key/data files).
+
+        Args:
+            db: Async database session.
+            folder: Filesystem folder name to match (e.g.
+                ``"Rooster Fighter (2026)"``).
+
+        Returns:
+            ``AnimeSeries`` instance or ``None`` if not found.
+        """
+        result = await db.execute(
+            select(AnimeSeries).where(AnimeSeries.folder == folder)
+        )
+        return result.scalar_one_or_none()
+
     @staticmethod
     async def get_all(
         db: AsyncSession,