fix: briefing discuss button — full article content + scroll + optimistic UI

Three bugs in discussArticle():
- Scroll selector was '.briefing-chat' (doesn't exist) → '.briefing-center';
  the panel never scrolled into view so the response was invisible until refresh
- Only 300-char snippet was sent to the LLM; now passes the full stored
  content (up to 2000 chars) from the backend
- User message wasn't shown until streaming ended; now added optimistically
  to messages[] immediately on click so it appears straight away

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

frontend/src/types/news.ts

@@ -3,6 +3,7 @@ export interface NewsItem {
   title: string
   url: string
   snippet: string
+  content: string
   published_at: string | null
   topics: string[]
   source: string