- Replace hardcoded error messages with structured error codes using exception.Builder. - Introduce new common error constants in exception/common.go (CommonErrorInvalidInput, CommonErrorUserNotFound, etc.). - Update exception/specific.go with domain-specific errors and remove redundant ones. - Apply consistent error handling across auth, event, user services and middleware. Co-authored-by: Gemini <gemini@google.com> Signed-off-by: Noa Virellia <noa@requiem.garden>