Remove legacy key file support after DB migration

- SerieScanner: Remove key file fallback, keep data file fallback
- SystemSettings: Add legacy_key_cleanup_completed flag
- initialization_service: Add cleanup task to remove key files from folders with DB entries
- Tests updated to reflect key file removal from legacy path

Key files caused duplicate key errors on folder rename. DB is now sole source of truth.

src/server/database/system_settings_service.py

@@ -155,6 +155,36 @@ class SystemSettingsService:
         await db.commit()
         logger.info("Marked legacy files migration as completed")
 
+    @staticmethod
+    async def is_legacy_key_cleanup_completed(db: AsyncSession) -> bool:
+        """Check if legacy key file cleanup has been completed.
+
+        Args:
+            db: Database session
+
+        Returns:
+            True if cleanup is completed, False otherwise
+        """
+        settings = await SystemSettingsService.get_or_create(db)
+        return settings.legacy_key_cleanup_completed
+
+    @staticmethod
+    async def mark_legacy_key_cleanup_completed(
+        db: AsyncSession,
+        timestamp: Optional[datetime] = None
+    ) -> None:
+        """Mark the legacy key file cleanup as completed.
+
+        Args:
+            db: Database session
+            timestamp: Optional timestamp to set, defaults to current time
+        """
+        settings = await SystemSettingsService.get_or_create(db)
+        settings.legacy_key_cleanup_completed = True
+        settings.last_scan_timestamp = timestamp or datetime.now(timezone.utc)
+        await db.commit()
+        logger.info("Marked legacy key file cleanup as completed")
+
     @staticmethod
     async def mark_initial_media_scan_completed(
         db: AsyncSession,
@@ -184,6 +214,8 @@ class SystemSettingsService:
         settings.initial_scan_completed = False
         settings.initial_nfo_scan_completed = False
         settings.initial_media_scan_completed = False
+        settings.migration_legacy_files_completed = False
+        settings.legacy_key_cleanup_completed = False
         settings.last_scan_timestamp = None
         await db.commit()
         logger.info("Reset all scan completion flags")