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

database/migrations/20260429180238_add_content_type_to_post.php

@@ -0,0 +1,36 @@
+<?php
+declare(strict_types=1);
+
+use think\migration\Migrator;
+use think\migration\db\Column;
+
+class AddContentTypeToPost extends Migrator
+{
+    /**
+     * Change Method.
+     *
+     * Write your reversible migrations using this method.
+     *
+     * More information on writing migrations is available here:
+     * http://docs.phinx.org/en/latest/migrations.html#the-abstractmigration-class
+     *
+     * The following commands can be used in this method and Phinx will
+     * automatically reverse them when rolling back:
+     *
+     *    createTable
+     *    renameTable
+     *    addColumn
+     *    renameColumn
+     *    addIndex
+     *    addForeignKey
+     *
+     * Remember to call "create()" or "update()" and NOT "save()" when working
+     * with the Table class.
+     */
+    public function change()
+    {
+        $this->table('post')->addColumn(
+            Column::make('content_type', 'string', ['limit' => 20, 'default' => 'html', 'comment' => '内容类型: html/markdown'])
+        )->update();
+    }
+}