feat: RSS embeddings, semantic news in chat, article-to-chat, richer briefings

- Embed RSS items at fetch time (nomic-embed-text); backfill at startup
- Semantic news search injected into chat system prompt ("Recent News You've Seen")
  when items match query above 0.55 cosine threshold (independent of note RAG)
- "Discuss in chat" button on news cards — creates a seeded conversation with
  the article title + full content, navigates directly to the new chat
- Briefing compilation now passes 500-char article excerpts (not just headlines)
  to the LLM and uses 8192 num_ctx to accommodate the larger prompt

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

frontend/src/api/client.ts

@@ -426,6 +426,10 @@ export async function deleteRssReaction(rssItemId: number): Promise<void> {
   return apiDelete(`/api/briefing/rss-reactions/${rssItemId}`);
 }
 
+export async function openArticleInChat(itemId: number): Promise<{ conversation_id: number }> {
+  return apiPost(`/api/chat/from-article/${itemId}`, {});
+}
+
 export async function geocodeAddress(address: string): Promise<{ lat: number; lon: number; display_name: string } | null> {
   try {
     const r = await apiPost<{ lat: number; lon: number; label: string }>('/api/briefing/weather/geocode', { query: address });