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/lib/services/saved_search_alert_service.dart

@@ -0,0 +1,147 @@
+import 'dart:async';
+
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter/foundation.dart';
+
+import 'discovery_area.dart';
+import 'notification_service.dart';
+import 'saved_searches_store.dart';
+import 'social_connection.dart';
+import 'social_service.dart' show SocialSession;
+import 'social_settings.dart';
+
+/// Builds the notification title for a fresh match on [search]. Injected so the
+/// app can supply a localized string while tests stay i18n-free.
+typedef AlertTitle = String Function(SavedSearch search);
+
+/// App-wide listener that turns incoming market offers into saved-search alerts
+/// (Wallapop's "búsquedas favoritas"). One discovery subscription over the
+/// SHARED [SocialConnection] for the user's whole current zone; each offer is
+/// matched locally against every saved search.
+///
+/// The discover stream replays a relay's stored offers before going live, so
+/// this doubles as the catch-up scan on app start — the per-search seen-key
+/// dedup in [SavedSearchesStore] keeps replays and edits silent, so only
+/// genuinely new matches notify. It re-subscribes when the connection
+/// reconnects and when the user's search area changes.
+///
+/// Foreground only — background/push delivery is a later, larger concern.
+class SavedSearchAlertService {
+  SavedSearchAlertService({
+    required SocialConnection connection,
+    required String selfPubkey,
+    required SavedSearchesStore store,
+    required SocialSettings settings,
+    NotificationService? notifications,
+    AlertTitle? alertTitle,
+  })  : _connection = connection,
+        _selfPubkey = selfPubkey,
+        _store = store,
+        _settings = settings,
+        _notifications = notifications,
+        _alertTitle = alertTitle ?? ((s) => s.label);
+
+  final SocialConnection _connection;
+  final String _selfPubkey;
+  final SavedSearchesStore _store;
+  final SocialSettings _settings;
+  final NotificationService? _notifications;
+  final AlertTitle _alertTitle;
+
+  StreamSubscription<SocialSession?>? _sessionsSub;
+  StreamSubscription<Offer>? _offersSub;
+  SocialSession? _session;
+  String _prefix = '';
+  bool _started = false;
+
+  /// Begins listening. Subscribe to the connection's sessions BEFORE it starts
+  /// connecting, so the first session is caught too.
+  void start() {
+    if (_started) return;
+    _started = true;
+    _sessionsSub = _connection.sessions.listen(_onSession);
+    final current = _connection.current;
+    if (current != null) _onSession(current);
+  }
+
+  /// Re-subscribes discovery, e.g. after the user changes their search area.
+  /// Cheap no-op when the session and prefix are unchanged.
+  Future<void> refreshArea() async {
+    final session = _session;
+    if (session != null) await _bind(session, force: true);
+  }
+
+  void _onSession(SocialSession? session) {
+    if (identical(session, _session) && _offersSub != null) return;
+    _session = session;
+    if (session == null) {
+      unawaited(_offersSub?.cancel());
+      _offersSub = null;
+      return;
+    }
+    unawaited(_bind(session, force: true));
+  }
+
+  /// (Re)binds the offer subscription to [session] over the user's current
+  /// zone. Skips re-subscribing when nothing changed unless [force].
+  Future<void> _bind(SocialSession session, {bool force = false}) async {
+    final area = await _settings.areaGeohash();
+    if (area.isEmpty) {
+      // No zone set yet → nothing to search. Drop any stale subscription.
+      await _offersSub?.cancel();
+      _offersSub = null;
+      _prefix = '';
+      return;
+    }
+    final prefix = searchPrefix(area, await _settings.searchPrecision());
+    if (!force && prefix == _prefix && _offersSub != null) return;
+    await _offersSub?.cancel();
+    _prefix = prefix;
+    _offersSub = session.offers
+        .discover(DiscoveryQuery(geohashPrefix: prefix))
+        .listen(
+          ingest,
+          onError: (_) {}, // handled by the connection's reconnect
+        );
+  }
+
+  /// Matches one incoming [offer] against every saved search and fires an alert
+  /// for each fresh match. A testable seam (no relay).
+  @visibleForTesting
+  Future<void> ingest(Offer offer) async {
+    // Never alert on your own listings.
+    if (offer.authorPubkeyHex == _selfPubkey) return;
+    // Only live offers are worth surfacing.
+    if (offer.status != OfferLifecycle.active) return;
+    // Respect the same moderation the market applies.
+    if (await _settings.isBlocked(offer.authorPubkeyHex)) return;
+    final hidden = await _settings.hiddenOfferKeys();
+    if (hidden.contains(SocialSettings.offerKey(offer.authorPubkeyHex, offer.id))) {
+      return;
+    }
+
+    final searches = await _store.list();
+    if (searches.isEmpty) return;
+    final key = SavedSearchesStore.keyOf(offer);
+    for (final search in searches) {
+      if (!search.matches(offer)) continue;
+      final isNew = await _store.markMatched(search.id, key);
+      if (isNew) {
+        await _notifications?.showSearchAlert(
+          searchId: search.id,
+          title: _alertTitle(search),
+        );
+      }
+    }
+  }
+
+  Future<void> stop() async {
+    _started = false;
+    await _sessionsSub?.cancel();
+    _sessionsSub = null;
+    await _offersSub?.cancel();
+    _offersSub = null;
+    _session = null;
+    _prefix = '';
+  }
+}