feat(practice-domain): add practice domains and let a clinic select one

A practice domain is the field a clinic operates in — beauty, dentistry —
and unlike Specialty it is configuration, not a label: treatment workflows
will bind to its code, so the code is immutable once created and only a
platform admin can mint one. A clinic that has not chosen a domain keeps
behaving exactly as it does today.

Assignment reuses PATCH /api/v1/clinic/{uuid} rather than adding a second
endpoint. An unknown domain uuid is rejected instead of silently dropped,
because a lost selection would only surface at the first protocol-driven
booking.

Also corrects ADR-0003: resource occupancy does not in fact guard the panel
booking path, which writes appointments.resource_id and no occupancy row at
all, so the doctor slot key cannot simply be dropped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/api/README.md

@@ -81,6 +81,7 @@ Only **digits** are translated — no characters are stripped, so `IR` in a sheb
 | [auth.md](auth.md) | Authentication — OTP, Login, JWT | 8 |
 | [doctor.md](doctor.md) | Doctor profile & addresses | 11 |
 | [clinic.md](clinic.md) | Clinics | 7 |
+| [practice-domain.md](practice-domain.md) | Practice domains — a clinic's field of practice | 3 |
 | [clinic-invitation.md](clinic-invitation.md) | Doctor invitations to clinics | 8 |
 | [resource.md](resource.md) | Resources, types, skills, pools | 16 |
 | [resource-calendar.md](resource-calendar.md) | Resource calendars, exceptions, national holidays | 9 |