feat(scanner): replace file writes with DB persistence for series

- SerieScanner.scan() now calls _persist_serie_to_db() instead of serie.save_to_file()
- Added _sync_episodes_to_db() helper to handle episode CRUD during sync
- EpisodeService gains delete_by_series() for targeted episode deletion
- SerieList gains add_to_db() async method for DB-based series addition
- test_serie_scanner_db_writes.py covers create/update/preserve/sync scenarios
- DATABASE.md updated with Series Persistence Flow section

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

src/server/database/service.py

@@ -649,11 +649,11 @@ class EpisodeService:
     @staticmethod
     async def delete(db: AsyncSession, episode_id: int) -> bool:
         """Delete episode.
-        
+
         Args:
             db: Database session
             episode_id: Episode primary key
-        
+
         Returns:
             True if deleted, False if not found
         """
@@ -662,6 +662,33 @@ class EpisodeService:
         )
         return result.rowcount > 0
 
+    @staticmethod
+    async def delete_by_series(
+        db: AsyncSession,
+        series_id: int,
+        season: int,
+        episode_number: int,
+    ) -> bool:
+        """Delete episode by series ID, season, and episode number.
+
+        Args:
+            db: Database session
+            series_id: Foreign key to AnimeSeries
+            season: Season number
+            episode_number: Episode number within season
+
+        Returns:
+            True if deleted, False if not found
+        """
+        result = await db.execute(
+            delete(Episode).where(
+                Episode.series_id == series_id,
+                Episode.season == season,
+                Episode.episode_number == episode_number,
+            )
+        )
+        return result.rowcount > 0
+
     @staticmethod
     async def delete_by_series_and_episode(
         db: AsyncSession,