feat: silent background polling for briefing screen

Timer.periodic every 60s calls silentRefresh() on BriefingNotifier.
silentRefresh() patches AsyncData directly — never triggers AsyncLoading —
so existing content stays on screen while the fetch is in flight.
WidgetsBindingObserver pauses polling when app is backgrounded.
Polling is also skipped while a reply is actively streaming.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/briefing/briefing_screen.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
@@ -16,15 +18,40 @@ class BriefingScreen extends ConsumerStatefulWidget {
   ConsumerState<BriefingScreen> createState() => _BriefingScreenState();
 }
 
-class _BriefingScreenState extends ConsumerState<BriefingScreen> {
+class _BriefingScreenState extends ConsumerState<BriefingScreen>
+    with WidgetsBindingObserver {
   final _controller = TextEditingController();
   final _scrollController = ScrollController();
   bool _refreshing = false;
   // rss_item_id -> 'up' | 'down' | null
   final Map<int, String?> _reactions = {};
 
+  Timer? _pollTimer;
+  bool _appInForeground = true;
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+    _pollTimer = Timer.periodic(const Duration(seconds: 60), (_) => _pollSilently());
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    _appInForeground = state == AppLifecycleState.resumed;
+  }
+
+  void _pollSilently() {
+    if (!_appInForeground || !mounted) return;
+    final isStreaming = ref.read(isBriefingStreamingProvider);
+    if (isStreaming) return;
+    ref.read(briefingProvider.notifier).silentRefresh();
+  }
+
   @override
   void dispose() {
+    _pollTimer?.cancel();
+    WidgetsBinding.instance.removeObserver(this);
     _controller.dispose();
     _scrollController.dispose();
     super.dispose();