Verify deletion actually removed the file before reporting success

MeTube's /delete only unlinks the file when it's configured with
DELETE_FILE_ON_TRASHCAN=true; otherwise it just drops the entry from its
own "done" list and returns {"status": "ok"} regardless -- we were trusting
that response and marking the job "deleted" (offering a re-download) while
the file was still sitting on mediaVM's disk the whole time.

Now HEAD-check the media_url right after the delete call. If the file is
still reachable, leave the job's status untouched (still "completed", still
playable) and surface a clear 409 explaining MeTube's own config is why,
rather than lying about local state.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/DownloadButton.tsx

@@ -75,6 +75,11 @@ function DownloadButton({ video }: Props) {
         >
           Удалить
         </button>
+        {deleteMutation.isError && (
+          <span className="download-badge error" title={(deleteMutation.error as Error).message}>
+            {(deleteMutation.error as Error).message}
+          </span>
+        )}
       </span>
     )
   }