feat(backup): add automatic weekly encrypted backups

A silent safety net against in-app data loss or DB corruption, with zero
friction. Driven off the app lifecycle (startup + when hidden), it writes a
sealed .tanemaki copy into the app's private storage once a week, keeping the
newest 3 (rotating). Reuses the manual backup's encryption via the extracted
ExportImportService.buildSealedBackup(); never throws, so a failed copy can't
crash startup or backgrounding. Not registered on web (no file storage).

Settings shows a quiet reassurance line with the date of the last automatic
copy (localised via intl), hidden where no automatic backup exists.

apps/app_seeds/lib/i18n/en.i18n.json

@@ -46,6 +46,9 @@
   },
   "backup": {
     "title": "Backup & restore",
+    "autoBackupTitle": "Automatic backups",
+    "autoBackupLast": "Last copy saved {date}",
+    "autoBackupNone": "A copy is kept automatically in the background",
     "exportJson": "Save a backup",
     "exportJsonSubtitle": "A complete copy to keep safe, restore later or move to another device",
     "importJson": "Restore a backup",