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/models/__init__.py

@@ -42,3 +42,4 @@ from fabledassistant.models.rss_feed import RssFeed, RssItem  # noqa: E402, F401
 from fabledassistant.models.weather_cache import WeatherCache  # noqa: E402, F401
 from fabledassistant.models.api_key import ApiKey  # noqa: E402, F401
 from fabledassistant.models.user_profile import UserProfile  # noqa: E402, F401
+from fabledassistant.models.rss_item_embedding import RssItemEmbedding  # noqa: E402, F401