Centralize backend error handling

[fuzziecoder]

Title

refactor: unify backend error responses and handling
 
Body
Current routes handle errors ad-hoc and return inconsistent response shapes. Introduce centralized error helpers.
 
Acceptance Criteria

• Common error format across all routes.
• Known errors map to correct status codes.
• Unknown errors return safe 500 response.
   

[AnushSingla]

@fuzziecoder assign it to me