Introduce the first per-appointment event system. On cancel (via the status
or general update endpoints) an AppointmentEvent (type=cancelled, «نوبت لغو
شد») is recorded with the actor, cancel time, and an optional cancel_reason,
plus a warning-level app_log entry. New GET /appointment/{uuid}/events
returns the ordered event list. The admin appointment detail page renders a
Timeline section and the cancel dialog now collects an optional reason.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>