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/upload.go

@@ -18,11 +18,13 @@ func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
 
 	files := r.MultipartForm.File["file"]
 	result, err := a.uploadService.CreateBox(files, services.UploadOptions{
-		MaxDays:      parseInt(r.FormValue("max_days")),
-		MaxDownloads: parseInt(r.FormValue("max_downloads")),
+		MaxDays:           parseInt(r.FormValue("max_days")),
+		MaxDownloads:      parseInt(r.FormValue("max_downloads")),
+		Password:          r.FormValue("password"),
+		ObfuscateMetadata: r.FormValue("obfuscate_metadata") == "on",
 	})
 	if err != nil {
-		a.logger.Warn("upload failed", "source", "user-upload", "code", 4001, "error", err.Error())
+		a.logger.Warn("upload failed", "source", "user-upload", "severity", "warn", "code", 4001, "error", err.Error())
 		helpers.WriteJSONError(w, http.StatusBadRequest, err.Error())
 		return
 	}