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 {