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.

frontend/src/pages/PostsPage.tsx

@@ -1,20 +0,0 @@
-import { getPosts } from '../api';
-import { useAsync } from '../lib/useAsync';
-import PostCard from '../components/PostCard';
-
-export default function PostsPage() {
-  const { data: posts, loading } = useAsync(() => getPosts(), []);
-  const list = posts ?? [];
-
-  return (
-    <div className="mx-auto max-w-5xl">
-      <h1 className="mb-6 text-3xl font-bold md:text-4xl">Archive</h1>
-      <p className="mb-10 text-lg text-primary-700">Every post, newest first.</p>
-      {loading && <p className="text-primary-500">Loading…</p>}
-      <div className="grid grid-cols-1 gap-6 md:grid-cols-2">
-        {list.map((post) => <PostCard key={post.slug} post={post} />)}
-      </div>
-      {!loading && list.length === 0 && <p className="text-primary-500">No posts yet.</p>}
-    </div>
-  );
-}