refactor: move transaction management from services to API endpoints

- Services now use db.flush() instead of db.commit() for database operations
- API endpoints handle transaction commit after service calls
- Remove db.rollback() from services (let exception handlers manage this)
- Ensures consistent transaction boundaries at API layer

This pattern gives API endpoints full control over when to commit,
allowing for better error handling and potential multi-operation transactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/shop/auth.py

@@ -83,6 +83,7 @@ def register_customer(
     customer = customer_service.register_customer(
         db=db, vendor_id=vendor.id, customer_data=customer_data
     )
+    db.commit()
 
     logger.info(
         f"New customer registered: {customer.email} for vendor {vendor.subdomain}",