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/vendor/marketplace.py

@@ -47,6 +47,7 @@ async def import_products_from_marketplace(
     import_job = marketplace_import_job_service.create_import_job(
         db, request, vendor, current_user
     )
+    db.commit()
 
     # Process in background
     background_tasks.add_task(