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/users.py

@@ -83,6 +83,7 @@ def create_user(
         role=user_data.role,
         current_admin_id=current_admin.id,
     )
+    db.commit()
 
     return UserDetailResponse(
         id=user.id,
@@ -175,6 +176,7 @@ def update_user(
         role=update_data.get("role"),
         is_active=update_data.get("is_active"),
     )
+    db.commit()
 
     return UserDetailResponse(
         id=user.id,
@@ -206,6 +208,7 @@ def toggle_user_status(
         user_id=user_id,
         current_admin_id=current_admin.id,
     )
+    db.commit()
 
     return UserStatusToggleResponse(message=message, is_active=user.is_active)
 
@@ -222,5 +225,6 @@ def delete_user(
         user_id=user_id,
         current_admin_id=current_admin.id,
     )
+    db.commit()
 
     return UserDeleteResponse(message=message)