feat(social): add SavedSearchesStore with alert bookkeeping

Keystore-backed, account-scoped store mirroring SavedOffersStore. Tracks
per-search seen keys (dedup, capped 500) and unread match keys (badge
source), with markMatched/markViewed/newMatchCount and a changes stream.

apps/app_seeds/test/services/saved_searches_store_test.dart

@@ -0,0 +1,97 @@
+import 'package:tane/services/saved_searches_store.dart';
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  late SavedSearchesStore store;
+
+  setUp(() => store = SavedSearchesStore(InMemorySecretStore()));
+  tearDown(() => store.close());
+
+  SavedSearch search(String id, {String query = '', int createdAt = 0}) =>
+      SavedSearch(id: id, label: id, query: query, createdAt: createdAt);
+
+  test('saves and lists newest first', () async {
+    await store.save(search('a', createdAt: 100));
+    await store.save(search('b', createdAt: 200));
+    final list = await store.list();
+    expect(list.map((s) => s.id), ['b', 'a']);
+  });
+
+  test('save is idempotent on id', () async {
+    await store.save(search('a', query: 'one'));
+    await store.save(search('a', query: 'two'));
+    final list = await store.list();
+    expect(list, hasLength(1));
+    expect(list.single.query, 'two');
+  });
+
+  test('remove deletes a search', () async {
+    await store.save(search('a'));
+    await store.remove('a');
+    expect(await store.list(), isEmpty);
+    expect(await store.exists('a'), isFalse);
+  });
+
+  test('markMatched reports a fresh match once, then dedups replays', () async {
+    await store.save(search('a'));
+    expect(await store.markMatched('a', 'author:o1'), isTrue);
+    // Same key again = replay/echo, not a new alert.
+    expect(await store.markMatched('a', 'author:o1'), isFalse);
+    expect(await store.markMatched('a', 'author:o2'), isTrue);
+    expect(await store.newMatchCount('a'), 2);
+  });
+
+  test('markMatched returns false for an unknown search', () async {
+    expect(await store.markMatched('ghost', 'author:o1'), isFalse);
+  });
+
+  test('seeded seen keys suppress alerts for already-visible offers', () async {
+    await store.save(search('a'), seedSeenKeys: ['author:o1']);
+    expect(await store.markMatched('a', 'author:o1'), isFalse);
+    expect(await store.newMatchCount('a'), 0);
+  });
+
+  test('markViewed clears unread but keeps seen (no re-alert)', () async {
+    await store.save(search('a'));
+    await store.markMatched('a', 'author:o1');
+    await store.markViewed('a');
+    expect(await store.newMatchCount('a'), 0);
+    // Still seen: the same offer won't alert again.
+    expect(await store.markMatched('a', 'author:o1'), isFalse);
+  });
+
+  test('totalNewMatchCount sums across searches', () async {
+    await store.save(search('a'));
+    await store.save(search('b'));
+    await store.markMatched('a', 'author:o1');
+    await store.markMatched('b', 'author:o2');
+    await store.markMatched('b', 'author:o3');
+    expect(await store.totalNewMatchCount(), 3);
+  });
+
+  test('changes stream fires on mutations', () async {
+    final events = <void>[];
+    final sub = store.changes.listen(events.add);
+    await store.save(search('a'));
+    await store.markMatched('a', 'author:o1');
+    await store.markViewed('a');
+    await store.remove('a');
+    await Future<void>.delayed(Duration.zero);
+    expect(events, hasLength(4));
+    await sub.cancel();
+  });
+
+  test('account scopes are isolated', () async {
+    final secret = InMemorySecretStore();
+    final s1 = SavedSearchesStore(secret, accountScope: 'acct1');
+    final s2 = SavedSearchesStore(secret, accountScope: 'acct2');
+    await s1.save(search('a'));
+    expect(await s1.list(), hasLength(1));
+    expect(await s2.list(), isEmpty);
+    await s1.close();
+    await s2.close();
+  });
+}