Add Error Handling Middleware

[NhanPhamThanh-IT]

Description

The boilerplate currently has basic routes and controllers, but lacks centralized error handling. To make the API more robust:

Tasks:

• Create an Express error‑handling middleware to catch synchronous and asynchronous errors.
• Ensure that unexpected errors are not leaking stack traces in production.
• Standardize error response format (status code, message, possibly error code).
• Possibly define custom error classes for common error types (NotFound, BadRequest, etc).
• Ensure validation errors (if added) are handled via this middleware.