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/app.py

@@ -261,6 +261,11 @@ def create_app() -> Quart:
                 await backfill_project_summaries()
             except Exception:
                 logger.warning("Project summary backfill failed", exc_info=True)
+            try:
+                from fabledassistant.services.embeddings import backfill_rss_item_embeddings
+                await backfill_rss_item_embeddings()
+            except Exception:
+                logger.warning("RSS embedding backfill failed", exc_info=True)
 
         asyncio.create_task(_delayed_backfill())