Port 4ducky to Rust (Axum + SQLite)

Same design, layout, URLs and features as the PHP version: browse grid,
image view with AJAX comments, BBCode, likes, uploads (file, URL and
bookmarklet), quicktagger, profile, block list, inbox, invites, Hall of
Ducks, Truhe, abuse reports and the admin tools.

- Parameterized SQL, escaped templates (askama), argon2 passwords,
  server-side sessions, CSRF tokens on every write, strict script CSP
- SSRF-safe remote image fetching, magic-byte upload checks
- Self-hosted captcha replaces reCAPTCHA v1; analytics/social widgets removed
- Vanilla JS replaces jQuery and its plugins with the same DOM behaviour
- Reverse proxy support (frontend_path, trust_proxy, secure_cookies)
- Admin CLI: admin, settings, invites

See README.md for running it and the full list of changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

templates/ajax/addon.html

@@ -0,0 +1,11 @@
+
+<p>
+	To use the remote upload just drag the following link somewhere to your bookmarks (or in Opera right click it
+	and choose <em>Bookmark link</em>):
+	&raquo; <a data-bookmarklet="1" href="javascript:void((function(){var%20e=document.createElement('script');e.setAttribute('type','text/javascript');e.setAttribute('src','{{ frontend_path }}media/post.js.php');document.body.appendChild(e)})());"><u>Post to {{ site_name }}</u></a> &laquo;
+</p>
+
+<p>
+	Now, if you're on a website where you see an image you want to post, just click on your newly created bookmark
+	and then on the image you want to post. Thats all. Really!
+</p>