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/pubspec.yaml

@@ -37,6 +37,8 @@ dependencies:
   # i18n (Weblate-friendly JSON per locale).
   slang: ^4.7.0
   slang_flutter: ^4.7.0
+  # ICU date/number formatting (locale-aware, never ad-hoc).
+  intl: ^0.20.2
 
   # Secure key storage (OS keystore).
   flutter_secure_storage: ^9.2.4