Add app_owner support for default privileges in permission granting

[nikoshet]

Changes:

• Extract app_owner username from TARGET_POSTGRES_URL environment variable
• Update default privileges logic:
  • If user is app_owner: use standard ALTER DEFAULT PRIVILEGES IN SCHEMA
  • If user is not app_owner: use ALTER DEFAULT PRIVILEGES FOR ROLE app_owner IN SCHEMA

This ensures that when app_owner creates new tables/functions/sequences,
application users automatically receive the appropriate permissions.