feat(content-type): add content_type field, markdown auto-conversion, and API doc updates

- Add content_type column to ul_post via migration
- Install league/commonmark for markdown->HTML conversion
- Add Post model accessors/setters for content_type and content
- Update API Articles controller save/update with content_type support
- Update API docs with content_type parameter and markdown example

Closes content-type-support plan

view/index/api_doc/index.html

@@ -196,6 +196,7 @@ X-API-Key: {api_key}</div>
                 "id": 1,
                 "title": "文章标题",
                 "content": "文章内容",
+                "content_type": "html",
                 "type": "1",
                 "status": 1,
                 "source": "api",
@@ -238,6 +239,7 @@ X-API-Key: {api_key}</div>
             "title": "文章标题",
             "content": "文章内容",
             "content_html": "&lt;p&gt;HTML内容&lt;/p&gt;",
+            "content_type": "html",
             "desc": "摘要",
             "poster": "/uploads/poster.jpg",
             "type": "1",
@@ -268,12 +270,13 @@ X-API-Key: {api_key}</div>
                                     <tr><td>poster</td><td>string</td><td>否</td><td>封面图 URL</td></tr>
                                     <tr><td>type</td><td>string</td><td>否</td><td>文章类型，默认 "1"</td></tr>
                                     <tr><td>status</td><td>int</td><td>否</td><td>状态，默认 0（草稿）</td></tr>
+                                    <tr><td>content_type</td><td>string</td><td>否</td><td>内容类型: "html"(默认) 或 "markdown"</td></tr>
                                     <tr><td>categorys</td><td>array</td><td>否</td><td>分类 ID 数组，如 [1, 2]</td></tr>
                                     <tr><td>tags</td><td>array</td><td>否</td><td>标签 ID 数组，如 [1, 2]</td></tr>
                                 </tbody>
                             </table>
 
-                            <h4>请求示例</h4>
+                            <h4>请求示例（HTML）</h4>
                             <div class="api-code">curl -X POST \
   "https://example.com/index.php/api/articles/save" \
   -H "Authorization: Bearer your_api_key" \
@@ -286,6 +289,21 @@ X-API-Key: {api_key}</div>
     "tags": [3]
   }'</div>
 
+                            <h4>请求示例（Markdown）</h4>
+                            <div class="api-code">curl -X POST \
+  "https://example.com/index.php/api/articles/save" \
+  -H "Authorization: Bearer your_api_key" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "title": "Markdown 文章",
+    "content": "# 标题\n\n段落内容",
+    "content_type": "markdown",
+    "type": "1",
+    "status": 1,
+    "categorys": [1, 2],
+    "tags": [3]
+  }'</div>
+
                             <h4>返回示例</h4>
                             <div class="api-code">{
     "code": 0,
@@ -319,6 +337,7 @@ X-API-Key: {api_key}</div>
                                     <tr><td>desc</td><td>string</td><td>否</td><td>文章摘要</td></tr>
                                     <tr><td>poster</td><td>string</td><td>否</td><td>封面图 URL</td></tr>
                                     <tr><td>status</td><td>int</td><td>否</td><td>状态</td></tr>
+                                    <tr><td>content_type</td><td>string</td><td>否</td><td>内容类型: "html"(默认) 或 "markdown"</td></tr>
                                     <tr><td>categorys</td><td>array</td><td>否</td><td>分类 ID 数组（全量覆盖）</td></tr>
                                     <tr><td>tags</td><td>array</td><td>否</td><td>标签 ID 数组（全量覆盖）</td></tr>
                                 </tbody>