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>

src/fabledassistant/services/llm.py

@@ -668,6 +668,33 @@ async def build_context(
                 + "\n--- End Included Notes ---"
             )
 
+    # Search for semantically relevant recent news items
+    try:
+        from fabledassistant.services.embeddings import get_embedding, semantic_search_rss_items
+        news_query_vec = await get_embedding(user_message)
+        news_hits = await semantic_search_rss_items(user_id, news_query_vec)
+        if news_hits:
+            news_snippets = []
+            for score, rss_item in news_hits:
+                feed_title = getattr(rss_item, "feed_title", "") or ""
+                excerpt = (rss_item.content or "")[:500].strip()
+                news_snippets.append(
+                    f"[{feed_title or 'News'}] {rss_item.title} (relevance: {round(score * 100)}%)\n"
+                    + (f"{excerpt}\n" if excerpt else "")
+                    + f"URL: {rss_item.url}"
+                )
+            system_parts.append(
+                "\n\n--- Recent News You've Seen ---\n"
+                + "\n\n".join(news_snippets)
+                + "\n--- End Recent News ---"
+            )
+            context_meta["rss_news"] = [
+                {"id": item.id, "title": item.title, "score": round(score, 2)}
+                for score, item in news_hits
+            ]
+    except Exception:
+        logger.debug("RSS semantic search skipped", exc_info=True)
+
     # Fetch URL content from user message
     urls = _find_urls(user_message)
     for url in urls[:2]:  # Limit to 2 URLs