refactor: reorganize startup persistence boundaries

tests/test_database_index_migration.py

@@ -76,7 +76,7 @@ from sqlalchemy.exc import IntegrityError
 
 from app.runtime.config import settings
 from app.db import get_engine
-from app.startup.database_initializer import init_db, update_db
+from app.startup.initializers.database import init_db, update_db
 
 media_tables = {media_tables!r}
 legacy_identity_columns = {legacy_identity_columns!r}