docs: update admin categories API with pagination parameters

- Add page/limit query parameters documentation
- Update response format to include pagination metadata
- Specify parameter constraints (page ≥ 1, limit 1-50)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/api-reference.md

@@ -931,10 +931,14 @@
 
 #### GET /admin/categories
 
-获取所有分类。
+获取分类列表。
 
 **认证**: Admin Token
 
+**查询参数**:
+- `page`: 页码 (默认: 1, 必须 ≥ 1)
+- `limit`: 每页数量 (默认: 20, 范围: 1-50)
+
 **响应**:
 ```json
 {
@@ -946,9 +950,15 @@
       "slug": "history",
       "parentId": null,
       "sortOrder": 1,
+      "questionCount": 120,
       "status": "active"
     }
   ],
+  "pagination": {
+    "total": 45,
+    "page": 1,
+    "limit": 20
+  },
   "error": null
 }
 ```