feat: wire discuss button in briefing RSS cards, cap cards at 3

Adds discussArticle() to BriefingApi and wires it through to the
RSS news cards in BriefingScreen so tapping Discuss opens a chat
conversation seeded with the article. Also caps RSS cards per
message at 3 to avoid overly long briefing threads.

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

lib/data/api/briefing_api.dart

@@ -70,6 +70,22 @@ class BriefingApi {
     }
   }
 
+  /// POST /api/briefing/articles/{itemId}/discuss  body: {"conv_id": convId}
+  /// Injects the article as context and triggers LLM generation.
+  /// Returns the assistant_message_id of the generating placeholder.
+  Future<int> discussArticle(int convId, int itemId) async {
+    try {
+      final response = await _dio.post(
+        '/api/briefing/articles/$itemId/discuss',
+        data: {'conv_id': convId},
+      );
+      final data = response.data as Map<String, dynamic>;
+      return data['assistant_message_id'] as int;
+    } on DioException catch (e) {
+      throw dioToApp(e);
+    }
+  }
+
   /// DELETE /api/briefing/rss-reactions/{rssItemId}
   Future<void> deleteRssReaction(int rssItemId) async {
     try {

lib/screens/briefing/briefing_screen.dart

@@ -93,6 +93,23 @@ class _BriefingScreenState extends ConsumerState<BriefingScreen>
     }
   }
 
+  Future<void> _handleDiscuss(int convId, int itemId) async {
+    try {
+      await ref.read(briefingProvider.notifier).discussArticle(convId, itemId);
+    } on AppException catch (e) {
+      if (mounted) {
+        ScaffoldMessenger.of(context)
+            .showSnackBar(SnackBar(content: Text(e.message)));
+      }
+    } catch (_) {
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(content: Text('Failed to start discussion.')),
+        );
+      }
+    }
+  }
+
   Future<void> _handleReaction(int itemId, String reaction) async {
     final current = _reactions[itemId];
     final next = current == reaction ? null : reaction;
@@ -268,8 +285,10 @@ class _BriefingScreenState extends ConsumerState<BriefingScreen>
                             final msg = conv.messages[i];
                             return _BriefingMessageItem(
                               message: msg,
+                              convId: conv.id,
                               reactions: _reactions,
                               onReaction: _handleReaction,
+                              onDiscuss: _handleDiscuss,
                             );
                           },
                         ),
@@ -370,13 +389,17 @@ class _BriefingScreenState extends ConsumerState<BriefingScreen>
 /// and RSS reaction buttons below it (for assistant messages with metadata).
 class _BriefingMessageItem extends StatelessWidget {
   final Message message;
+  final int convId;
   final Map<int, String?> reactions;
   final void Function(int itemId, String reaction) onReaction;
+  final void Function(int convId, int itemId) onDiscuss;
 
   const _BriefingMessageItem({
     required this.message,
+    required this.convId,
     required this.reactions,
     required this.onReaction,
+    required this.onDiscuss,
   });
 
   @override
@@ -388,11 +411,11 @@ class _BriefingMessageItem extends StatelessWidget {
     final bool hasWeatherKey = isAssistant && meta != null && meta.containsKey('weather');
     final weatherData = hasWeatherKey ? meta['weather'] as Map<String, dynamic>? : null;
 
-    // RSS news cards
+    // RSS news cards — cap at 3
     final rssItemsRaw = isAssistant && meta != null
         ? (meta['rss_items'] as List<dynamic>?)?.cast<Map<String, dynamic>>() ?? []
         : <Map<String, dynamic>>[];
-    final rssItems = rssItemsRaw.map(RssItemMeta.fromJson).toList();
+    final rssItems = rssItemsRaw.map(RssItemMeta.fromJson).take(3).toList();
 
     return Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -408,6 +431,7 @@ class _BriefingMessageItem extends StatelessWidget {
                 item: item,
                 reaction: reactions[item.id],
                 onReaction: onReaction,
+                onDiscuss: () => onDiscuss(convId, item.id),
               )).toList(),
             ),
           ),