book() now accepts for_self plus patient_* fields: for_self fills the
patient from the paying user's profile, otherwise patient_name/mobile are
required (422 if missing) and the rest are stored. Every booking starts
pending with a 15-minute expires_at. Persisting goes through
bookAtomically (re-check inside a transaction) so two concurrent requests
for the same slot can't both win — the loser gets 409.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
