feat: weather card and RSS reactions in briefing screen

- Parse metadata field on Message model (weather, rss_item_ids)
- Add WeatherCard widget: location, current temp, condition, today hi/lo,
  delta from yesterday, scrollable 3-day forecast strip
- BriefingScreen shows WeatherCard above the first assistant message that
  carries weather metadata (mirrors web BriefingView.vue behaviour)
- RSS reaction buttons (👍/👎) shown below assistant messages with
  rss_item_ids; optimistic toggle with rollback on failure
- Add postRssReaction / deleteRssReaction to BriefingApi

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

lib/data/models/message.dart

@@ -7,6 +7,7 @@ class Message {
   final String content;
   final String status; // "complete" | "generating"
   final DateTime? createdAt;
+  final Map<String, dynamic>? metadata;
 
   const Message({
     this.id,
@@ -15,6 +16,7 @@ class Message {
     required this.content,
     this.status = 'complete',
     this.createdAt,
+    this.metadata,
   });
 
   factory Message.fromJson(Map<String, dynamic> json) => Message(
@@ -26,6 +28,7 @@ class Message {
         createdAt: json['created_at'] != null
             ? DateTime.parse(json['created_at'] as String)
             : null,
+        metadata: json['metadata'] as Map<String, dynamic>?,
       );
 
   Message copyWith({String? content, String? status}) => Message(
@@ -35,5 +38,6 @@ class Message {
         content: content ?? this.content,
         status: status ?? this.status,
         createdAt: createdAt,
+        metadata: metadata,
       );
 }