[GSSoC26] feat: Add consistent API error response format middleware

[ashrion]

Description

This PR adds a consistent API error response format across all endpoints.

Changes

• Created backend/utils/apiError.js with ApiError class and factory methods
• Updated backend/middleware/errorHandler.js to use consistent error format
• Standardized error responses: { success, message, errors, timestamp }
• Added asyncHandler wrapper for automatic async error catching
• Include stack traces in development mode only
• Handle common error types: ValidationError, CastError, duplicate keys, JWT errors

Motivation

• Error responses were inconsistent across different endpoints
• Some returned { success: false, message, errors }, others { message }, others { error }
• Consistent format improves frontend error handling and debugging

Testing

• Trigger validation error and verify response format
• Trigger 404 and verify format
• Trigger JWT error and verify format
• Verify stack trace only appears in development

Closes #456

GSSoC26 contribution