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>

frontend/src/components/ChatPanel.vue

@@ -299,12 +299,22 @@ defineExpose({ focus, prefill, send })
       <!-- Message list -->
       <div ref="messagesEl" class="messages-container">
         <div class="messages-inner">
-          <ChatMessage
-            v-for="msg in store.currentConversation?.messages ?? []"
+          <template
+            v-for="(msg, index) in store.currentConversation?.messages ?? []"
             :key="msg.id"
-            :message="msg"
-            @save-as-note="handleSaveAsNote"
-          />
+          >
+            <!-- Briefing slot separator: shown before a 2nd+ briefing slot message -->
+            <div
+              v-if="briefingMode && index > 0 && msg.role === 'assistant' && msg.metadata?.rss_item_ids"
+              class="briefing-slot-separator"
+            >
+              <span>New Briefing Update</span>
+            </div>
+            <ChatMessage
+              :message="msg"
+              @save-as-note="handleSaveAsNote"
+            />
+          </template>
           <!-- Streaming bubble -->
           <ChatStreamingBubble v-if="store.streaming" />
           <!-- Queued messages -->
@@ -523,6 +533,28 @@ defineExpose({ focus, prefill, send })
   justify-content: flex-end;
 }
 
+/* Briefing slot separator */
+.briefing-slot-separator {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  margin: 1.25rem 0 0.5rem;
+  color: var(--color-text-muted, #888);
+  font-size: 0.7rem;
+  font-weight: 600;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  opacity: 0.7;
+}
+.briefing-slot-separator::before,
+.briefing-slot-separator::after {
+  content: '';
+  flex: 1;
+  height: 1px;
+  background: var(--color-border, #333);
+  opacity: 0.5;
+}
+
 /* Context sidebar */
 .context-sidebar {
   width: 200px;

frontend/src/types/chat.ts

@@ -28,6 +28,7 @@ export interface Message {
   context_note_id: number | null;
   context_note_title?: string | null;
   tool_calls?: ToolCallRecord[] | null;
+  metadata?: Record<string, unknown> | null;
   created_at: string;
   timing?: GenerationTiming;
   thinking?: string;