Add admin UI + MinIO image uploads

- Admin API: list/edit posts (incl. drafts, raw markdown), moderate comments, subscribers,
  author bio/links, and presigned image upload to MinIO (bucket is public-read for covers).
- Admin UI at /admin: post editor with cover-image upload, comment moderation, subscriber list.
- Public security switched to authenticated-paths so static assets (/images, /data) stay public.

.gitignore

@@ -1,7 +0,0 @@
-target/
-frontend/node_modules/
-frontend/dist/
-.idea/
-*.iml
-.vscode/
-.DS_Store