feat(social): add SavedSearchAlertService + notification alerts channel

One discovery subscription over the shared connection for the user's
current zone; matches each incoming offer against saved searches and
fires a text-free OS alert on the new 'alerts' channel for fresh matches.
Skips own/blocked/hidden/non-active offers and dedups via seen keys. The
replay-on-connect doubles as the catch-up scan. Notification payloads are
now prefixed so taps route to chat or the saved search.

apps/app_seeds/test/services/saved_search_alert_service_test.dart

@@ -0,0 +1,148 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/notification_service.dart';
+import 'package:tane/services/saved_search_alert_service.dart';
+import 'package:tane/services/saved_searches_store.dart';
+import 'package:tane/services/social_connection.dart';
+import 'package:tane/services/social_service.dart';
+import 'package:tane/services/social_settings.dart';
+
+import '../support/test_support.dart';
+
+/// Records search-alert calls instead of touching the OS plugin.
+class _RecordingNotifications extends NotificationService {
+  _RecordingNotifications() : super(supported: false);
+
+  final searchIds = <String>[];
+  final titles = <String>[];
+
+  @override
+  Future<void> showSearchAlert({
+    required String searchId,
+    required String title,
+  }) async {
+    searchIds.add(searchId);
+    titles.add(title);
+  }
+}
+
+void main() {
+  const seedHex =
+      '000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f';
+  const myPubkey = 'me';
+
+  // A connection that never actually opens — ingest doesn't use it.
+  Future<SocialConnection> offlineConnection() async => SocialConnection(
+        social: await SocialService.fromRootSeedHex(seedHex),
+        settings: SocialSettings(InMemorySecretStore()),
+        open: (_) async => throw StateError('unused in ingest tests'),
+        online: const Stream.empty(),
+      );
+
+  late SavedSearchesStore store;
+  late SocialSettings settings;
+  late _RecordingNotifications notifications;
+  late SavedSearchAlertService service;
+
+  setUp(() async {
+    store = SavedSearchesStore(InMemorySecretStore());
+    settings = SocialSettings(InMemorySecretStore());
+    notifications = _RecordingNotifications();
+    service = SavedSearchAlertService(
+      connection: await offlineConnection(),
+      selfPubkey: myPubkey,
+      store: store,
+      settings: settings,
+      notifications: notifications,
+      alertTitle: (s) => 'New match: ${s.label}',
+    );
+  });
+
+  tearDown(() async {
+    await service.stop();
+    await store.close();
+  });
+
+  Offer offer({
+    String id = 'o1',
+    String author = 'grower',
+    String summary = 'Heirloom tomato seeds',
+    OfferType type = OfferType.gift,
+    OfferLifecycle status = OfferLifecycle.active,
+    String? category = 'Solanaceae',
+    bool isOrganic = false,
+  }) =>
+      Offer(
+        id: id,
+        authorPubkeyHex: author,
+        summary: summary,
+        type: type,
+        status: status,
+        approxGeohash: 'u09',
+        category: category,
+        isOrganic: isOrganic,
+      );
+
+  SavedSearch search(String id, {String query = ''}) =>
+      SavedSearch(id: id, label: id, query: query);
+
+  test('a matching offer fires one alert and records the match', () async {
+    await store.save(search('tom', query: 'tomato'));
+    await service.ingest(offer());
+    expect(notifications.searchIds, ['tom']);
+    expect(notifications.titles, ['New match: tom']);
+    expect(await store.newMatchCount('tom'), 1);
+  });
+
+  test('a non-matching offer is silent', () async {
+    await store.save(search('pep', query: 'pepper'));
+    await service.ingest(offer());
+    expect(notifications.searchIds, isEmpty);
+  });
+
+  test('a replayed offer does not re-alert', () async {
+    await store.save(search('tom', query: 'tomato'));
+    await service.ingest(offer());
+    await service.ingest(offer()); // relay replays the stored event
+    expect(notifications.searchIds, ['tom']);
+  });
+
+  test('one offer can match several searches', () async {
+    await store.save(search('all'));
+    await store.save(search('tom', query: 'tomato'));
+    await service.ingest(offer());
+    expect(notifications.searchIds, containsAll(<String>['all', 'tom']));
+    expect(notifications.searchIds, hasLength(2));
+  });
+
+  test('my own offers never alert', () async {
+    await store.save(search('all'));
+    await service.ingest(offer(author: myPubkey));
+    expect(notifications.searchIds, isEmpty);
+  });
+
+  test('non-active offers never alert', () async {
+    await store.save(search('all'));
+    await service.ingest(offer(status: OfferLifecycle.closed));
+    expect(notifications.searchIds, isEmpty);
+  });
+
+  test('offers from a blocked author never alert', () async {
+    await settings.block('grower');
+    await store.save(search('all'));
+    await service.ingest(offer());
+    expect(notifications.searchIds, isEmpty);
+  });
+
+  test('a hidden (reported) offer never alerts', () async {
+    await settings.hideOffer(authorPubkeyHex: 'grower', offerId: 'o1');
+    await store.save(search('all'));
+    await service.ingest(offer());
+    expect(notifications.searchIds, isEmpty);
+  });
+
+  test('with no saved searches nothing fires', () async {
+    await service.ingest(offer());
+    expect(notifications.searchIds, isEmpty);
+  });
+}