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/main.dart

@@ -5,6 +5,7 @@ import 'data/species_repository.dart';
 import 'data/variety_repository.dart';
 import 'di/injector.dart';
 import 'i18n/strings.g.dart';
+import 'services/auto_backup_service.dart';
 import 'services/onboarding_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
@@ -23,6 +24,9 @@ Future<void> main() async {
         social: getIt<SocialService>(),
         socialSettings: getIt<SocialSettings>(),
         showIntro: !await onboarding.introSeen(),
+        autoBackup: getIt.isRegistered<AutoBackupService>()
+            ? getIt<AutoBackupService>()
+            : null,
       ),
     ),
   );