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/libs/handlers/pages.go

@@ -11,7 +11,7 @@ type homeData struct {
 }
 
 func (a *App) Home(w http.ResponseWriter, r *http.Request) {
-	a.renderer.Render(w, http.StatusOK, "home.gohtml", web.PageData{
+	a.renderer.Render(w, http.StatusOK, "home.html", web.PageData{
 		Title:       "Upload your files",
 		Description: "Upload and share files through a self-hosted Warpbox instance.",
 		Data: homeData{