fix(backup): persist last-success/error status; wall-clock-based schedule

BackupService tracked last-success/last-error as plain in-process fields
and scheduled the daily backup via setInterval measured from process
start — so any server restart (deploy/crash/OOM/reboot, routine under
`restart: always`) silently reset the admin UI to "last successful
backup: Never" and drifted the actual cadence, independent of whether
backups were writing correctly to disk (they were — a real field
incident at park-buzi showed 7 valid rotating backups on disk with the
status stuck on "Never").

Persist last-success/error to new site_config columns (migration 0025)
and add BackupService.isDue(), computed from the persisted timestamp
instead of process uptime; server.ts now polls every 15 min and lets
isDue() gate the actual run. No API/UI contract change.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

packages/db/drizzle/meta/_journal.json

@@ -176,6 +176,13 @@
       "when": 1783948800000,
       "tag": "0024_validation_programs",
       "breakpoints": true
+    },
+    {
+      "idx": 25,
+      "version": "6",
+      "when": 1788078414270,
+      "tag": "0025_backup_last_status",
+      "breakpoints": true
     }
   ]
 }