- Create domain/errors.py as canonical error hierarchy: DomainError, InvalidStateTransition, PermissionViolation, BusinessRuleViolation, EntityNotFoundError, DuplicateEntityError
- InvalidOperationError now inherits from BusinessRuleViolation for semantic consistency
- Convert domain/exceptions.py to backward-compatible re-export shim with legacy aliases (DomainException, InvalidTransitionError, AuthorizationError)
- Update error_handler.py to import from domain/errors.py and map all new error types
- Update main.py to register DomainError (new base) as the exception handler root
