[Backend] Consolidate Error Handling with Custom Error Classes

[devJaja]

Problem

Error handling across the backend is inconsistent:

• Some routes throw generic Error objects
• Others return res.status(500).json({ error: 'message' }) directly
• The error handler in backend/src/api/middleware/errorHandler.ts only handles known error shapes
• No distinction between client errors (4xx) and server errors (5xx)

Current State

Looking at error handling in routes and coordinator:

// Some places:
throw new Error('Task not found');

// Others:
return res.status(500).json({ error: 'Internal error' });

// Error handler assumes:
if (err instanceof AppError) { ... }

Proposed Solution

1. Create backend/src/errors/ directory with:
   • AppError.ts — base class with statusCode, isOperational, code
   • ValidationError.ts — 400 with field details
   • NotFoundError.ts — 404 with resource type/id
   • UnauthorizedError.ts — 401/403
   • RateLimitError.ts — 429
2. Update error handler to recognize all custom error types
3. Add error.code for machine-readable error identification
4. Ensure all errors are operational (no stack traces in production responses)

Acceptance Criteria

• Custom error classes in backend/src/errors/
• Error handler handles all custom types
• No raw Error objects thrown in routes
• Error responses have consistent shape: { error: { code, message, details? } }
• Stack traces hidden in production
• All existing error tests pass

Difficulty

High — touches every route and middleware

Labels

backend enhancement complex

[Netty-kun]

Hello team, I'd like to take ownership of this task. I've gone through the requirements and acceptance criteria and am prepared to implement the necessary fix.

[grantfox-oss]

🦊 GrantFox — @Netty-kun has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #109)
2. Your PR will be reviewed by the Epta-Node maintainers

Good luck! Track your progress on GrantFox.