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/preview.html

@@ -0,0 +1,36 @@
+{{define "preview.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="download-view" aria-labelledby="preview-title">
+  <div class="card download-card">
+    <div class="card-content">
+      {{if .Data.Locked}}
+        <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="preview-title">Protected file</h1>
+        <p class="download-subtitle">Unlock the box before viewing this file.</p>
+        <a class="button button-primary button-wide" href="/d/{{.Data.Box.ID}}">Unlock box</a>
+      {{else}}
+        <div class="preview-stage">
+          {{if eq .Data.File.PreviewKind "image"}}
+            <img src="{{.Data.DownloadURL}}?inline=1" alt="{{.Data.File.Name}}">
+          {{else if eq .Data.File.PreviewKind "video"}}
+            <video src="{{.Data.DownloadURL}}?inline=1" poster="{{.Data.File.ThumbnailURL}}" controls preload="metadata"></video>
+          {{else if eq .Data.File.PreviewKind "audio"}}
+            <audio src="{{.Data.DownloadURL}}?inline=1" controls preload="metadata"></audio>
+          {{else}}
+            <img src="{{.Data.File.ThumbnailURL}}" alt="">
+          {{end}}
+        </div>
+        <h1 id="preview-title">{{.Data.File.Name}}</h1>
+        <p class="download-subtitle">{{.Data.File.Size}} · {{.Data.File.ContentType}}</p>
+        <a class="button button-primary button-wide" href="{{.Data.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 file
+        </a>
+      {{end}}
+    </div>
+  </div>
+</section>
+{{end}}