feat(admin): implement provider-specific storage configuration pages

Refactor the admin storage backend creation and editing flows to use
provider-specific pages (e.g., `/admin/storage/new/sftp`) instead of a
single generic form. This ensures only relevant fields are rendered for
each storage provider (such as SFTP, S3, or WebDAV).

Additionally:
- Prevent mutation of the storage provider type during backend edits.
- Add comprehensive unit tests for provider-specific rendering, edit
  validation, and CSRF/admin route protection.

backend/libs/jobs/cleanup.go

@@ -26,6 +26,10 @@ func newCleanupJob(cfg config.Config, logger *slog.Logger, uploadService *servic
 	}
 }
 
+func RunCleanupNow(uploadService *services.UploadService, logger *slog.Logger) (int, error) {
+	return cleanupUnavailableBoxes(uploadService, logger)
+}
+
 func cleanupUnavailableBoxes(uploadService *services.UploadService, logger *slog.Logger) (int, error) {
 	boxes, err := uploadService.ListBoxes(0)
 	if err != nil {