pythonvCard4 parses vCard BDAY fields into datetime.date objects, but
the Contact model expects Optional[str]. This caused a validation error
that crashed the entire contact list. Convert at the client layer
(consistent with the calendar client pattern) with a defensive check
at the server mapping layer.
Closes#672
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>