feat: Phase 8.5 — ConversationSummary 查询优化，支持 SQL 聚合获取消息统计

backend/internal/session/memory.go

@@ -187,12 +187,13 @@ func (m *MemoryManager) UpdateTitle(_ context.Context, sessionID string, title s
 }
 
 // ListByUser 获取用户的对话列表（分页，按 UpdatedAt 降序）。
-func (m *MemoryManager) ListByUser(_ context.Context, userID string, page, size int) ([]ConversationSummary, int, error) {
+// 若配置了 MessageRepository，消息统计从 PostgreSQL 聚合查询（更准确）。
+func (m *MemoryManager) ListByUser(ctx context.Context, userID string, page, size int) ([]ConversationSummary, int, error) {
 	m.mu.RLock()
-	defer m.mu.RUnlock()
 
 	// 收集该用户的所有 session
 	var list []ConversationSummary
+	var sessionIDs []string
 	for _, entry := range m.sessions {
 		if entry.session.UserID != userID {
 			continue
@@ -201,15 +202,32 @@ func (m *MemoryManager) ListByUser(_ context.Context, userID string, page, size
 			continue
 		}
 		summary := ConversationSummary{
-			ID:           entry.session.ID,
-			Title:        entry.session.Title,
-			MessageCount: len(entry.history),
-			UpdatedAt:    entry.lastActive,
+			ID:        entry.session.ID,
+			Title:     entry.session.Title,
+			UpdatedAt: entry.lastActive,
 		}
+		// 先用内存值填充，后续可能被 PG 统计覆盖
+		summary.MessageCount = len(entry.history)
 		if len(entry.history) > 0 {
 			summary.LastMessage = entry.history[len(entry.history)-1].Content
 		}
 		list = append(list, summary)
+		sessionIDs = append(sessionIDs, entry.session.ID)
+	}
+	m.mu.RUnlock()
+
+	// 若配置了 msgRepo，从 PostgreSQL 获取更准确的消息统计
+	if m.msgRepo != nil && len(sessionIDs) > 0 {
+		if stats, err := m.msgRepo.GetSessionMessageStats(ctx, sessionIDs); err == nil {
+			for i := range list {
+				if s, ok := stats[list[i].ID]; ok {
+					list[i].LastMessage = s.LastMessage
+					list[i].MessageCount = s.MessageCount
+				}
+			}
+		} else {
+			logger.Log.Warnw("get session message stats failed, falling back to in-memory", "error", err)
+		}
 	}
 
 	// 按 UpdatedAt 降序排序