feat(briefing): add article Discuss endpoint with synthetic tool exchange

POST /api/briefing/articles/<id>/discuss injects stored article content
as a persisted read_article tool exchange before triggering generation.
The LLM sees the article as already read; follow-ups retain context via
the fixed history builder. Frontend Discuss button now calls the new
endpoint instead of inlining article text in the user message.

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

frontend/src/views/BriefingView.vue

@@ -7,6 +7,7 @@ import WeatherCard from '@/components/WeatherCard.vue'
 import BriefingSetupWizard from '@/components/BriefingSetupWizard.vue'
 import {
   apiGet,
+  apiPost,
   getBriefingConfig,
   getBriefingConversations,
   getBriefingToday,
@@ -113,26 +114,19 @@ watch(selectedConvId, async (id) => {
   }
 })
 
-// Keep send() as a helper for discussArticle
-async function send(text: string) {
-  if (!text || !todayConvId.value || chatStore.streaming) return
-  if (chatStore.currentConversation?.id !== todayConvId.value) {
-    await chatStore.fetchConversation(todayConvId.value)
-  }
-  await chatStore.sendMessage(text, undefined, undefined, true)
-}
-
 async function discussArticle(item: NewsItem) {
-  if (!todayConvId.value) return
+  if (!todayConvId.value || chatStore.streaming) return
   if (!isToday.value) selectedConvId.value = todayConvId.value
-  const body = (item.content || item.snippet || '').trim()
-  const bodyBlock = body ? `\n\n---\n${body}\n---` : ''
-  const text = `Please summarize and share your thoughts on this article. Do not use any research or search tools — respond conversationally based only on the content below.\n\n**${item.title}**\nSource: ${item.source}${bodyBlock}`
-  // Scroll to chat column
   await nextTick(() => {
     document.querySelector('.briefing-center')?.scrollIntoView({ behavior: 'smooth', block: 'nearest' })
   })
-  await send(text)
+  try {
+    await apiPost<{ assistant_message_id: number }>(`/api/briefing/articles/${item.id}/discuss`, { conv_id: todayConvId.value })
+  } catch {
+    return
+  }
+  await chatStore.fetchConversation(todayConvId.value)
+  await chatStore.reconnectIfGenerating(todayConvId.value)
 }
 
 // RSS reactions: map of rss_item_id -> 'up' | 'down' | null