feat(article-discuss): unify /news + briefing entry points, persist summaries to RAG

Both the /news discuss button and the briefing discuss button now call a
shared seed_article_discussion() helper that stages the synthetic
read_article tool exchange and the conversational seed prompt — behavior
stays byte-identical across entry points. /news also auto-starts
generation so the chat screen lands on an in-flight stream.

First assistant reply in a seeded article conversation is persisted as a
Note (tags: article-summary + article topics) and backlinked via
rss_items.discussion_note_id, so the knowledge base stops being amnesiac
about articles the user has engaged with.

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

src/fabledassistant/services/chat.py

@@ -187,6 +187,7 @@ async def add_message(
     context_note_id: int | None = None,
     status: str | None = None,
     tool_calls: list | None = None,
+    msg_metadata: dict | None = None,
 ) -> Message:
     async with async_session() as session:
         kwargs: dict = dict(
@@ -199,6 +200,8 @@ async def add_message(
             kwargs["status"] = status
         if tool_calls is not None:
             kwargs["tool_calls"] = tool_calls
+        if msg_metadata is not None:
+            kwargs["msg_metadata"] = msg_metadata
         msg = Message(**kwargs)
         session.add(msg)
         # Touch conversation updated_at