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

app/model/Post.php

@@ -156,6 +156,30 @@ class Post extends Base
         return trim($value);
     }
 
+    public function getContentTypeAttr($value)
+    {
+        return $value ?: 'html';
+    }
+
+    public function setContentTypeAttr($value)
+    {
+        if (!in_array($value, ['html', 'markdown'], true)) {
+            throw new \InvalidArgumentException('Invalid content_type: ' . $value);
+        }
+
+        return $value;
+    }
+
+    public function setContentAttr($value)
+    {
+        return trim($value);
+    }
+
+    public function getContentAttr($value)
+    {
+        return $value;
+    }
+
     public function getContentMarkdownAttr()
     {
         $content_html = $this->getAttr('content_html');