phsl / new-api

- Improve error handling in DeleteCustomOAuthProvider to log and return errors when fetching binding counts.
- Refactor user creation and OAuth binding logic to use transactions for atomic operations, ensuring data integrity.
- Add unique constraints to UserOAuthBinding model to prevent duplicate bindings.
- Enhance GitHub OAuth provider error logging for non-200 responses.
- Update AccountManagement component to provide clearer error messages on API failures.