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.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/main.tsx

@@ -1,13 +0,0 @@
-import { StrictMode } from 'react';
-import { createRoot } from 'react-dom/client';
-import { BrowserRouter } from 'react-router-dom';
-import App from './App';
-import './index.css';
-
-createRoot(document.getElementById('root')!).render(
-  <StrictMode>
-    <BrowserRouter>
-      <App />
-    </BrowserRouter>
-  </StrictMode>,
-);