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/vendor_domains.py

@@ -68,6 +68,7 @@ def add_vendor_domain(
     domain = vendor_domain_service.add_domain(
         db=db, vendor_id=vendor_id, domain_data=domain_data
     )
+    db.commit()
 
     return VendorDomainResponse(
         id=domain.id,
@@ -186,6 +187,7 @@ def update_vendor_domain(
     domain = vendor_domain_service.update_domain(
         db=db, domain_id=domain_id, domain_update=domain_update
     )
+    db.commit()
 
     return VendorDomainResponse(
         id=domain.id,
@@ -224,6 +226,7 @@ def delete_vendor_domain(
 
     # Delete domain
     message = vendor_domain_service.delete_domain(db, domain_id)
+    db.commit()
 
     return DomainDeletionResponse(
         message=message, domain=domain_name, vendor_id=vendor_id
@@ -259,6 +262,7 @@ def verify_domain_ownership(
     - 502: DNS query failed
     """
     domain, message = vendor_domain_service.verify_domain(db, domain_id)
+    db.commit()
 
     return DomainVerificationResponse(
         message=message,