🔴 CRITICAL: Add global exception handler

[GitAddRemote]

Problem

No consistent error handling:

• No @ControllerAdvice
• JwtAuthFilter silently swallows exceptions
• Inconsistent error responses

Solution

Create GlobalExceptionHandler with @RestControllerAdvice:

• Handle MethodArgumentNotValidException
• Handle AccessDeniedException
• Handle custom business exceptions
• Return structured ApiError responses

Files to Create

• libs/common/.../ApiError.java
• apps/backend/.../GlobalExceptionHandler.java
• Custom exception classes

Files to Modify

• JwtAuthFilter.java (fix exception handling)
• AuthController.java (structured responses)

Acceptance Criteria

• GlobalExceptionHandler with @RestControllerAdvice
• Structured error response format
• Consistent HTTP status codes
• Tests for error scenarios

Priority

🔴 Critical - Required for production operations