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/vite.config.ts

@@ -1,19 +0,0 @@
-import { defineConfig } from 'vite';
-import react from '@vitejs/plugin-react';
-import tailwindcss from '@tailwindcss/vite';
-
-// dist/ is copied into the Spring jar's static/ by platform-parent's managed build.
-export default defineConfig({
-  base: '/',
-  plugins: [react(), tailwindcss()],
-  build: { outDir: 'dist', emptyOutDir: true },
-  server: {
-    port: 5173,
-    proxy: {
-      '/api': 'http://127.0.0.1:8080',
-      '/oauth2': 'http://127.0.0.1:8080',
-      '/login': 'http://127.0.0.1:8080',
-      '/logout': 'http://127.0.0.1:8080',
-    },
-  },
-});