feat: add admin console, cleanup, and thumbnail workers

- Implement a token-authenticated admin console at `/admin` with overview metrics and file management.
- Add a background worker to periodically clean up expired boxes based on `WARPBOX_CLEANUP_EVERY`.
- Add a background worker to generate image and video thumbnails based on `WARPBOX_THUMBNAIL_EVERY`.
- Update file storage paths to use `@each@` and `@thumb@` prefixes to separate original files from thumbnails.
- Add severity fields to startup logs and update configuration templates.

backend/templates/pages/admin_login.html

@@ -0,0 +1,23 @@
+{{define "admin_login.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="download-view" aria-labelledby="admin-login-title">
+  <form class="card download-card" action="/admin/login" method="post">
+    <div class="card-content">
+      <div class="file-emblem" aria-hidden="true">
+        <svg viewBox="0 0 24 24" role="img" focusable="false"><path d="M12 17a2 2 0 1 0 0-4 2 2 0 0 0 0 4Z" /><path d="M19 11V8A7 7 0 0 0 5 8v3" /><path d="M5 11h14v10H5z" /></svg>
+      </div>
+      <h1 id="admin-login-title">Admin login</h1>
+      <p class="download-subtitle">Use the token from <code>WARPBOX_ADMIN_TOKEN</code>.</p>
+      {{if .Data.Error}}<p class="form-error">{{.Data.Error}}</p>{{end}}
+      <div class="unlock-form">
+        <label>
+          <span>Admin token</span>
+          <input type="password" name="token" autocomplete="current-password" required>
+        </label>
+        <button class="button button-primary" type="submit">Sign in</button>
+      </div>
+    </div>
+  </form>
+</section>
+{{end}}