feat: agent proactively re-engages users on returning sessions

Two complementary changes to reduce conversation cold-start friction:

A. Returning-user re-greeting (backend)
- When WS reconnects to a non-empty conversation and last_message_at is older
  than chat_re_greeting_idle_hours (default 6h), the agent emits a warm
  continuation message that references prior history instead of staying silent.
- Self-debouncing: the AI message updates last_message_at, so reconnects
  within the window will not re-trigger.
- Skipped while profile collection is still pending.

D. Topic suggestion chips (backend + Expo)
- New WS message type topic_suggestions carries 3-4 quick-start chips derived
  from the current memoir stage's empty slots (deterministic, no extra LLM
  cost). Sent alongside opening / re-greeting / resume.
- Expo chat screen renders a horizontally-scrollable chip row above the input
  bar; tapping a chip sends the chip's text as a user message and clears the
  row. Sending any text/voice also clears the chips.

app-expo/src/features/conversation/hooks.ts

@@ -2,7 +2,7 @@ import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import { File, Paths } from 'expo-file-system';
 import { useCallback, useEffect, useRef, useState } from 'react';
 
-import type { WsConnectionState } from '@/core/ws/types';
+import type { TopicSuggestion, WsConnectionState } from '@/core/ws/types';
 
 import { conversationApi } from './api';
 import { conversationMessagesRepository } from './conversation-messages-repository';
@@ -183,6 +183,9 @@ interface RealtimeSessionState {
   /** 已发出用户消息，尚未收到助手首段流式文本（用于「正在回复」气泡） */
   awaitingAssistantReply: boolean;
   error: string | null;
+  /** 服务端下发的 quick-start 话题 chips；用户首次发文本/语音后清空 */
+  topicSuggestions: TopicSuggestion[];
+  dismissTopicSuggestions: () => void;
   sendText: (text: string) => void;
   sendVoiceMessage: (uri: string, durationMs: number) => Promise<boolean>;
   sendEndConversation: () => void;
@@ -204,6 +207,9 @@ export function useRealtimeSession({
     useState<StreamingAgentMessage | null>(null);
   const [awaitingAssistantReply, setAwaitingAssistantReply] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [topicSuggestions, setTopicSuggestions] = useState<TopicSuggestion[]>(
+    [],
+  );
 
   const handleStreamingText: StreamingTextCallback = useCallback(
     (text, isComplete) => {
@@ -225,6 +231,17 @@ export function useRealtimeSession({
     setError(message);
   }, []);
 
+  const handleTopicSuggestions = useCallback(
+    (payload: { suggestions: TopicSuggestion[] }) => {
+      setTopicSuggestions(payload.suggestions);
+    },
+    [],
+  );
+
+  const dismissTopicSuggestions = useCallback(() => {
+    setTopicSuggestions([]);
+  }, []);
+
   useEffect(() => {
     if (!enabled || !conversationId) return;
 
@@ -233,6 +250,7 @@ export function useRealtimeSession({
       queryClient,
       onStreamingText: handleStreamingText,
       onTtsSegment,
+      onTopicSuggestions: handleTopicSuggestions,
       onError: handleError,
       onStateChange: setConnectionState,
     });
@@ -246,6 +264,7 @@ export function useRealtimeSession({
       setConnectionState('disconnected');
       setStreamingMessage(null);
       setAwaitingAssistantReply(false);
+      setTopicSuggestions([]);
     };
   }, [
     conversationId,
@@ -253,6 +272,7 @@ export function useRealtimeSession({
     queryClient,
     handleStreamingText,
     handleError,
+    handleTopicSuggestions,
     onTtsSegment,
   ]);
 
@@ -267,6 +287,7 @@ export function useRealtimeSession({
       }
 
       setAwaitingAssistantReply(true);
+      setTopicSuggestions([]);
       onTtsPlaybackResume?.();
 
       const localId = `pending_${Date.now()}`;
@@ -326,6 +347,7 @@ export function useRealtimeSession({
         }
 
         setAwaitingAssistantReply(true);
+        setTopicSuggestions([]);
         const localId = `pending_voice_${Date.now()}`;
         await voiceSegmentStore.recordSentSegment({
           voiceSessionId,
@@ -385,6 +407,8 @@ export function useRealtimeSession({
     streamingMessage,
     awaitingAssistantReply,
     error,
+    topicSuggestions,
+    dismissTopicSuggestions,
     sendText,
     sendVoiceMessage,
     sendEndConversation,