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/routes/chat.py

@@ -502,3 +502,52 @@ async def delete_model_route():
     except Exception as e:
         logger.warning("Failed to delete model %s: %s", model_name, e)
         return jsonify({"error": str(e)}), 500
+
+
+@chat_bp.route("/from-article/<int:item_id>", methods=["POST"])
+@login_required
+async def create_conversation_from_article(item_id: int):
+    """Create a chat conversation seeded with an RSS article's content."""
+    from sqlalchemy import select as _select
+    from fabledassistant.models import async_session as _async_session
+    from fabledassistant.models.rss_feed import RssItem, RssFeed
+
+    uid = get_current_user_id()
+
+    async with _async_session() as session:
+        result = await session.execute(
+            _select(RssItem, RssFeed.title.label("feed_title"))
+            .join(RssFeed, RssItem.feed_id == RssFeed.id)
+            .where(RssItem.id == item_id, RssFeed.user_id == uid)
+        )
+        row = result.first()
+
+    if row is None:
+        return jsonify({"error": "Article not found"}), 404
+
+    item, feed_title = row
+
+    conv_title = (item.title or "Article discussion")[:80]
+    conv = await create_conversation(uid, title=conv_title, conversation_type="chat")
+
+    source = feed_title or "News"
+    content_body = (item.content or "").strip()
+    seeded_text = f"**{source}**\n\n**{item.title}**"
+    if content_body:
+        seeded_text += f"\n\n{content_body}"
+    if item.url:
+        seeded_text += f"\n\nSource: {item.url}"
+
+    from fabledassistant.models.conversation import Message
+    from fabledassistant.models import async_session as _session2
+    async with _session2() as session:
+        msg = Message(
+            conversation_id=conv.id,
+            role="assistant",
+            content=seeded_text,
+            msg_metadata={"rss_item_ids": [item_id]},
+        )
+        session.add(msg)
+        await session.commit()
+
+    return jsonify({"conversation_id": conv.id}), 201