Prevent Registration Race Conditions (409 Conflict)

[viceda-s]

Problem: AuthService.register() suffers from a Time-of-Check to Time-of-Use (TOCTOU) race condition, resulting in generic 500 errors if unique constraints are violated concurrently.

Acceptance Criteria:

• Update GlobalExceptionHandler to explicitly catch DataIntegrityViolationException.
• Map unique constraint violations (specifically on the users table email column) to a 409 Conflict HTTP response.
• Adopt RFC 7807 (ProblemDetail) for all error responses instead of custom maps.

[viceda-s]

Handled registration race conditions by invoking userRepository.saveAndFlush() and catching DataIntegrityViolationException.