feat: the Backups screen says whether backups are keeping up, and the System view warns when they aren't

- A Backups row on Settings → System and the nav dot turn amber when the
  newest database dump is more than a day old or failed, or the weekly
  bucket check found files missing that no media run has sent since.
  No row when backups aren't set up.
- The weekly bucket check is kept as a run (kind "reconcile") with its
  counts, trimmed to the last 12, and listed on the Backups card.
- The Backups card leads with the same verdict and the newest dump's size,
  shows bytes beside the running media job's file count, and lets each run
  open its list of failed files with why.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -192,6 +192,11 @@ fingerprint, under `media/`. It never deletes from the bucket; the bucket's own
 retention thins old dumps. The first copy of a large library takes hours; a
 restart along the way carries on where it stopped rather than starting over.
 
+Once a week the bucket is checked against what was sent, and anything missing
+goes again in the next run. When the newest database backup is more than a day
+old or failed, or the check found files missing, the **Backups** row on
+**Settings → System** and the dot beside the brand turn amber and say why.
+
 To rebuild after losing the server, start a fresh install (first boot as
 above), enter the same destination, and under **Restore from the bucket** choose
 a dump. The database is replaced, then every file is fetched back to its place