Refactor: Remove brittle, string-based exception checking in AuthService

[fred-maina]

Description

In AuthService.signUp, database errors are caught and handled by checking the exception message for specific constraint names:

} catch (DataIntegrityViolationException e) {
    if (e.getMessage().contains("users_email_key")) {
        authResponse.setMessage("Email already exists");
    } else if (e.getMessage().contains("idx_user_username") || e.getMessage().contains("users_username_key")) {
        authResponse.setMessage("Username already exists");
    } //...
}

This is extremely brittle. If the database schema changes, a constraint is renamed, or a different database driver is used, these checks will fail silently, and the user will get a generic "Data integrity violation" message.

Acceptance Criteria

1. Remove all e.getMessage().contains(...) checks for database constraints in AuthService.java.
2. Refactor the signUp method to be more proactive. Before attempting to save, perform explicit checks using the repository:
   • if (userRepository.existsByEmail(request.getEmail())) { ... }
   • if (userRepository.existsByUsernameIgnoreCase(request.getUsername())) { ... }
3. This moves the logic from fragile, reactive exception handling to robust, proactive validation.
4. This logic should be applied to setUsername as well, which already does a better job but can be cleaned up.