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/content_pages.py

@@ -191,6 +191,7 @@ def create_vendor_page(
         display_order=page_data.display_order,
         created_by=current_user.id,
     )
+    db.commit()
 
     return page.to_dict()
 
@@ -226,6 +227,7 @@ def update_vendor_page(
         display_order=page_data.display_order,
         updated_by=current_user.id,
     )
+    db.commit()
 
     return page.to_dict()
 
@@ -247,3 +249,4 @@ def delete_vendor_page(
 
     # Delete with ownership check in service layer
     content_page_service.delete_vendor_page(db, page_id, current_user.vendor_id)
+    db.commit()