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/admin/vendors.py

@@ -50,6 +50,7 @@ def create_vendor(
     The vendor inherits owner and contact information from its parent company.
     """
     vendor = admin_service.create_vendor(db=db, vendor_data=vendor_data)
+    db.commit()
 
     return VendorCreateResponse(
         # Vendor fields
@@ -204,6 +205,7 @@ def update_vendor(
     """
     vendor = vendor_service.get_vendor_by_identifier(db, vendor_identifier)
     vendor = admin_service.update_vendor(db, vendor.id, vendor_update)
+    db.commit()
     return _build_vendor_detail_response(vendor)
 
 
@@ -303,4 +305,5 @@ def delete_vendor(
 
     vendor = vendor_service.get_vendor_by_identifier(db, vendor_identifier)
     message = admin_service.delete_vendor(db, vendor.id)
+    db.commit()
     return {"message": message}