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

composer.json

@@ -33,7 +33,8 @@
         "topthink/think-filesystem": "^2.0",
         "mibe/feedwriter": "^1.1",
         "matomo/device-detector": "^6.1",
-        "intervention/image": "^2.7"
+        "intervention/image": "^2.7",
+        "league/commonmark": "^2.8"
     },
     "require-dev": {
         "symfony/var-dumper": "^4.2"