fix: actually call migration function (was only importing)

Bug: db_init.py was importing migrate_to_postgres but not calling main().
Result: Migration appeared successful but no data was migrated (0 users).

Fix: Import and call migrate_to_postgres.main()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/db_init.py

@@ -119,8 +119,8 @@ if __name__ == "__main__":
         print("  Starting automatic migration...")
         # Import and run migration
         try:
-            import migrate_to_postgres
-            print("✓ Migration completed")
+            from migrate_to_postgres import main as migrate
+            migrate()
         except Exception as e:
             print(f"✗ Migration failed: {e}")
             sys.exit(1)