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/clinic.md

@@ -193,10 +193,19 @@ Update a clinic.
 | `uuid` | string (UUID) | Clinic UUID |
 
 ### Request Body
-Same fields as POST — all optional.
+Same fields as POST — all optional — plus:
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `practice_domain_uuid` | string (UUID) \| `""` \| `null` | حوزهٔ فعالیت کلینیک. رشتهٔ خالی یا `null` یعنی «پاک کن»؛ نبودنِ کلید یعنی «دست نزن». uuid ناشناس ۴۲۲ می‌گیرد، نه رد شدن بی‌صدا. ← [practice-domain.md](./practice-domain.md) |
 
 ### Response `200`
-Updated clinic object (same structure as GET).
+Updated clinic object (same structure as GET). Carries `practice_domain` — the full domain object, or
+`null` when unset:
+
+```json
+{"uuid":"8c7bfd18-9159-11f1-b98b-f28fd8aa5db5","code":"beauty","name":"کلینیک زیبایی","sort_order":0,"active":true}
+```
 
 ### Errors
 | Code | HTTP | Description |
@@ -204,6 +213,7 @@ Updated clinic object (same structure as GET).
 | `ERR_AUTH_001` | 401 | Missing token |
 | `ERR_FORBIDDEN_001` | 403 | Not the owner |
 | `ERR_NOT_FOUND_001` | 404 | Clinic not found |
+| `ERR_VALIDATION_002` | 422 | `practice_domain_uuid` به هیچ حوزه‌ای اشاره نمی‌کند |
 
 ---