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>

frontend/test/components/backupRunsTable.spec.js

@@ -0,0 +1,43 @@
+// @vitest-environment happy-dom
+import { describe, it, expect } from 'vitest'
+
+import BackupRunsTable from '../../src/components/settings/BackupRunsTable.vue'
+import { mountComponent } from '../support/mountComponent.js'
+
+const MEDIA_RUN = {
+  id: 7, kind: 'media', status: 'ok', started_at: new Date().toISOString(),
+  duration_seconds: 90, size_bytes: 2048,
+  error: "3 file(s) weren't sent; the next run tries them again.",
+  offsite: { bucket: 'curator', endpoint: 'http://archives.example', key: 'media/' },
+  manifest: {
+    failed: 3,
+    failures: [
+      { path: '/images/a/one.jpg', error: 'HTTP 503: busy.' },
+      { path: '/images/b/two.zip', error: 'OSError: gone' },
+    ],
+  },
+}
+
+describe('BackupRunsTable', () => {
+  it('offers a run\'s failed files, counted in full', () => {
+    const w = mountComponent(BackupRunsTable, { props: { runs: [MEDIA_RUN] } })
+    expect(w.text()).toContain('3 files failed')
+    expect(w.text()).not.toContain('/images/a/one.jpg')
+  })
+
+  it('lists each failed file with why once opened, and says how many more', async () => {
+    const w = mountComponent(BackupRunsTable, { props: { runs: [MEDIA_RUN] } })
+    await w.find('v-btn').trigger('click')
+    const t = w.text()
+    expect(t).toContain('/images/a/one.jpg')
+    expect(t).toContain('HTTP 503: busy.')
+    expect(t).toContain('/images/b/two.zip')
+    expect(t).toContain('and 1 more.')
+  })
+
+  it('shows no failure button on a clean run', () => {
+    const clean = { ...MEDIA_RUN, id: 8, error: null, manifest: { failed: 0, failures: [] } }
+    const w = mountComponent(BackupRunsTable, { props: { runs: [clean] } })
+    expect(w.text()).not.toContain('failed')
+  })
+})