feat(specialty): public doctor-counts endpoint (per city)

Add GET /api/v1/specialties/doctor-counts?city_id= returning every active
specialty with number_of_doctors (distinct doctors via doctor_specialties,
scoped by doctor_cities when city_id is given). Make /api/v1/specialties GET
public. Powers the /specialties page count. Docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/api/specialty.md

@@ -42,6 +42,39 @@ List all medical specialties.
 
 ---
 
+## GET `/api/v1/specialties/doctor-counts`
+
+List active specialties together with the number of doctors in each — optionally scoped to a city. Used by the public `/specialties` page to show «N پزشک» under each specialty.
+
+**Permission:** `PUBLIC`
+
+### Query Parameters
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `city_id` | integer | ❌ | Count only doctors in this city (the `categories.id` of the city). Omit for a global count. |
+
+### Response `200`
+```json
+{
+  "success": true,
+  "data": [
+    {
+      "id": 2,
+      "name": "داخلی عمومی",
+      "slug": "...",
+      "parent_id": null,
+      "status": "active",
+      "weight": 10,
+      "number_of_doctors": 10
+    }
+  ]
+}
+```
+- Returns **all** active specialties; those with no doctors have `number_of_doctors: 0`.
+- `number_of_doctors` counts distinct doctors joined through `doctor_specialties` (and `doctor_cities` when `city_id` is given), matching the public `GET /api/v1/doctors?specialty_id=&city_id=` filter.
+
+---
+
 ## GET `/api/v1/admin/specialties`
 
 List all specialties with pagination (admin view — includes inactive).