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/download.html

@@ -0,0 +1,70 @@
+{{define "download.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="download-view download-view-wide" aria-labelledby="download-title">
+  <div class="card download-card">
+    <div class="card-content">
+      <div class="file-emblem" aria-hidden="true">
+        <svg viewBox="0 0 24 24" role="img" focusable="false"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8Z" /><path d="M14 2v6h6" /></svg>
+      </div>
+      <h1 id="download-title">{{if .Data.Locked}}Protected box{{else}}Download files{{end}}</h1>
+      <p class="download-subtitle">Bucket id: {{.Data.Box.ID}}</p>
+
+      {{if .Data.Locked}}
+        <form class="unlock-form" action="/d/{{.Data.Box.ID}}/unlock" method="post">
+          <label>
+            <span>Password</span>
+            <input type="password" name="password" autocomplete="current-password" required>
+          </label>
+          <button class="button button-primary" type="submit">Unlock box</button>
+        </form>
+        {{if .Data.Obfuscated}}
+          <p class="download-subtitle">File names, counts, and previews are hidden until the password is entered.</p>
+        {{end}}
+      {{end}}
+
+      {{if .Data.Files}}
+        <div class="badge-row">
+          <span class="badge">Expires {{.Data.ExpiresLabel}}</span>
+          {{if .Data.MaxDownloads}}<span class="badge">{{.Data.DownloadCount}} / {{.Data.MaxDownloads}} downloads</span>{{end}}
+        </div>
+
+        {{if not .Data.Locked}}
+          <a class="button button-primary button-wide" href="{{.Data.ZipURL}}">
+            <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M12 3v12m0 0 4-4m-4 4-4-4M5 21h14" /></svg>
+            Download zip
+          </a>
+        {{end}}
+
+        <div class="view-toolbar" aria-label="File view options">
+          <button class="button button-outline is-active" type="button" data-view-button="list">List</button>
+          <button class="button button-outline" type="button" data-view-button="thumbs">Thumbnails</button>
+          <button class="button button-outline" type="button" data-preview-images>Preview images only</button>
+        </div>
+
+        <div class="download-list file-browser is-list" data-file-browser>
+          {{range .Data.Files}}
+            <article class="download-item file-card" data-kind="{{.PreviewKind}}">
+              <a class="thumb-link" href="{{.URL}}" aria-label="Preview {{.Name}}">
+                <img src="{{.ThumbnailURL}}" alt="" loading="lazy">
+              </a>
+              <a class="file-main" href="{{.URL}}">
+                <strong>{{.Name}}</strong>
+                <small>{{.Size}} · {{.ContentType}}</small>
+              </a>
+              {{if not $.Data.Locked}}
+                <a class="button button-outline" href="{{.DownloadURL}}">
+                  <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M12 3v12m0 0 4-4m-4 4-4-4M5 21h14" /></svg>
+                  Download
+                </a>
+              {{end}}
+            </article>
+          {{end}}
+        </div>
+      {{else if not .Data.Locked}}
+        <p class="download-subtitle">{{.Data.ExpiresLabel}}</p>
+      {{end}}
+    </div>
+  </div>
+</section>
+{{end}}