fix(briefing): use briefing context for follow-ups; add slot separator

- build_context: when conversation_type is 'briefing', inject a system
  prompt instruction telling the model to answer from conversation history
  and article context instead of searching the web
- Consolidate briefing conversation type detection to one DB query (was
  being checked twice — once for the system prompt addition, once for
  article context injection)
- ChatPanel: render a visual 'New Briefing Update' separator line before
  2nd+ briefing slot messages (identified by metadata.rss_item_ids)
- types/chat.ts: add metadata field to Message interface

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

src/fabledassistant/services/llm.py

@@ -607,6 +607,25 @@ async def build_context(
             f"\n\n--- Earlier Conversation ---\n{history_summary}\n--- End Earlier Conversation ---"
         )
 
+    # Detect briefing conversation — used for both system prompt instruction and article injection
+    _is_briefing_conv = False
+    if conv_id is not None:
+        from fabledassistant.models import async_session as _async_session
+        from fabledassistant.models.conversation import Conversation as _Conversation
+        async with _async_session() as _sess:
+            _conv = await _sess.get(_Conversation, conv_id)
+        if _conv and getattr(_conv, "conversation_type", None) == "briefing":
+            _is_briefing_conv = True
+
+    if _is_briefing_conv:
+        system_content += (
+            "\n\nYou are in a briefing conversation. "
+            "The conversation history contains today's briefing — news stories, weather, and tasks. "
+            "When the user asks about a topic, person, or event from the briefing, answer directly "
+            "from the conversation history and the article context that follows. "
+            "Do NOT search the web for information that is already present in the briefing."
+        )
+
     context_meta: dict = {
         "context_note_id": None,
         "context_note_title": None,
@@ -769,16 +788,10 @@ async def build_context(
             )
 
     # Briefing article context for follow-up Q&A
-    if conv_id is not None:
-        from fabledassistant.models import async_session as _async_session
-        from fabledassistant.models.conversation import Conversation
-
-        async with _async_session() as _sess:
-            _conv = await _sess.get(Conversation, conv_id)
-        if _conv and getattr(_conv, "conversation_type", None) == "briefing":
-            article_context = await _build_briefing_article_context(conv_id)
-            if article_context:
-                user_context_parts.append(article_context.strip())
+    if _is_briefing_conv:
+        article_context = await _build_briefing_article_context(conv_id)  # type: ignore[arg-type]
+        if article_context:
+            user_context_parts.append(article_context.strip())
 
     # Build final user message — context prefix (if any) followed by the actual message
     if user_context_parts: