feat: add UniqueConstraintViolationException

[michalsn]

Description
This PR adds:

• UniqueConstraintViolationException (extends DatabaseException), thrown on duplicate key violations across all database drivers
• $db->getLastException(): ?DatabaseException - when DBDebug is false, exceptions are not thrown. This method provides a reliable way to inspect the type of failure, since raw error codes from $db->error()['code'] differ across drivers
• DatabaseException::getDatabaseCode(): int|string - exposes the native driver error code on the exception itself, as getCode() is constrained to int by PHP Throwable interface and cannot carry string SQLSTATE codes

getLastException() is intentionally a transitional API. The long-term direction is to drop DBDebug = false entirely (v5 ?), leaving exceptions as the only error signalling mechanism - consistent with how every other major PHP framework handles database errors.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide