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/src/schema.ts

@@ -288,6 +288,20 @@ export const siteConfig = sqliteTable("site_config", {
   backupKeepLast: integer("backup_keep_last"),
   /** Beyond keepLast, keep one backup per day for this many days. null ⇒ code default (30). */
   backupKeepDailyDays: integer("backup_keep_daily_days"),
+  /** ISO timestamp of the last backup that actually completed successfully. Persisted here
+   *  (not just in-process memory) so the admin UI's "last successful backup" survives a
+   *  server restart — before this column existed, a restart silently reset that status to
+   *  "Never" even with valid backups already on disk. null = no successful run recorded yet.
+   *  See wiki/concepts/backup-recovery.md. */
+  backupLastSuccessAt: text("backup_last_success_at"),
+  /** JSON-encoded { path, bytes, prunedFiles } of the last successful run, for the same
+   *  restart-durability reason as backupLastSuccessAt. null = none recorded yet. */
+  backupLastResultJson: text("backup_last_result_json"),
+  /** ISO timestamp of the last FAILED scheduled/manual backup attempt, persisted for the same
+   *  reason. null = no failure recorded (or none since the last success). */
+  backupLastErrorAt: text("backup_last_error_at"),
+  /** Error message of the last failed attempt. Cleared (set null) on the next success. */
+  backupLastError: text("backup_last_error"),
   updatedAt: text("updated_at")
     .notNull()
     .default(sql`(current_timestamp)`),