feat: add legacy key/data file migration to database

- Add migration_legacy_files_completed flag to SystemSettings model
- Create legacy_file_migration service to migrate series from key/data files
- Integrate legacy migration into initialization_service startup flow
- Add integration tests for legacy file migration
- Update DATABASE.md documentation with migration details
- Fix various test and service issues (nfo_repair, tmdb_client, download_service)
- Add test_database_schema unit tests

src/server/database/system_settings_service.py

@@ -125,6 +125,36 @@ class SystemSettingsService:
         settings = await SystemSettingsService.get_or_create(db)
         return settings.initial_media_scan_completed
 
+    @staticmethod
+    async def is_migration_legacy_files_completed(db: AsyncSession) -> bool:
+        """Check if legacy key/data file migration has been completed.
+        
+        Args:
+            db: Database session
+            
+        Returns:
+            True if legacy migration is completed, False otherwise
+        """
+        settings = await SystemSettingsService.get_or_create(db)
+        return settings.migration_legacy_files_completed
+
+    @staticmethod
+    async def mark_migration_legacy_files_completed(
+        db: AsyncSession,
+        timestamp: Optional[datetime] = None
+    ) -> None:
+        """Mark the legacy key/data file migration as completed.
+        
+        Args:
+            db: Database session
+            timestamp: Optional timestamp to set, defaults to current time
+        """
+        settings = await SystemSettingsService.get_or_create(db)
+        settings.migration_legacy_files_completed = True
+        settings.last_scan_timestamp = timestamp or datetime.now(timezone.utc)
+        await db.commit()
+        logger.info("Marked legacy files migration as completed")
+
     @staticmethod
     async def mark_initial_media_scan_completed(
         db: AsyncSession,