Add a Proxmox Backup Server integration: datastore/snapshot verification status

Proxmox VE already shows whether the last vzdump push to PBS succeeded, but
has no visibility into PBS's own backup verification, GC/prune health, or
host status. This adds PBS as its own integration (own adapter, page, nav
entry, and Dashboard widget) that reads datastore usage and, for every
stored snapshot, its verification state directly from PBS.

A new daily check (mirroring the existing Proxmox backup-failure check)
notifies when a snapshot has failed verification or a datastore couldn't be
read, with its own toggle in Settings -> Notifications and its own
maintenance-window silencing.

Not verified against a live PBS instance — built from PBS's published API
docs and a scratch test against a mocked PBS server exercising the adapter's
parsing and auth-header format (PBSAPIToken uses a colon separator, unlike
PVE's PVEAPIToken which uses =). See INTEGRATIONS.md for details and the
"not verified" caveat.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/pages/Maintenance.tsx

@@ -351,6 +351,7 @@ export default function Maintenance({ user }: { user: CurrentUser }) {
                 <li>Server offline and disk-full alerts (for a server).</li>
                 <li>Storage, volume and disk-health alerts (for a Proxmox or Synology integration).</li>
                 <li>Backup-failure and uncovered-guest alerts (for a Proxmox integration).</li>
+                <li>Backup-verification-failure alerts (for a Proxmox Backup Server integration).</li>
                 <li>Failed-run alerts for the Semaphore or Gitea integration you choose.</li>
                 <li>
                   "Integration down" alerts for that <em>type</em> of service — these are tracked per type (e.g. all