feat(history): 会话列表lastMessage预览(B3) — 单查询无N+1

- history/session_preview.go: SessionMeta.LastMessage + 单条SQL(LEFT JOIN取每会话ORDER BY created_at DESC,rowid DESC LIMIT 1,走覆盖索引)
- ListSessions 迁出重写(签名不变), content折叠空白截前60rune
- gateway: historySessionSummary 增 lastMessage{role,content,createdAt}(omitempty), /sessions与/admin/sessions共用
- 新增测试(session_preview_test, history_lastmessage_test)
- Docs/api-client.md v0.22 同步

internal/services/gateway/history.go

@@ -20,6 +20,18 @@ type historySessionSummary struct {
 	Type      string `json:"type"` // "main" | "sub"
 	Title     string `json:"title,omitempty"`
 	CreatedAt int64  `json:"createdAt"`
+	// LastMessage is a display-only preview of the session's newest message, so the
+	// client can render a conversation row (e.g. the Android session list) without a
+	// per-session request. It is omitted for a session with no messages. Content is
+	// already truncated by the store.
+	LastMessage *historyLastMessage `json:"lastMessage,omitempty"`
+}
+
+// historyLastMessage is the wire shape of the last-message preview in a session summary.
+type historyLastMessage struct {
+	Role      string `json:"role"` // "user" | "assistant"
+	Content   string `json:"content"`
+	CreatedAt int64  `json:"createdAt"`
 }
 
 // historyMessage is the wire shape of one persisted chat message in the
@@ -60,15 +72,25 @@ func (s *Server) handleHistorySessions(w http.ResponseWriter, r *http.Request) {
 	out := make([]historySessionSummary, 0, len(meta))
 	for _, m := range meta {
 		out = append(out, historySessionSummary{
-			ID:        m.ID,
-			Type:      m.Type,
-			Title:     m.Title,
-			CreatedAt: m.CreatedAt,
+			ID:          m.ID,
+			Type:        m.Type,
+			Title:       m.Title,
+			CreatedAt:   m.CreatedAt,
+			LastMessage: toHistoryLastMessage(m.LastMessage),
 		})
 	}
 	writeJSON(w, http.StatusOK, out)
 }
 
+// toHistoryLastMessage maps a stored last-message preview to its wire shape, returning
+// nil for a session that has no messages (so the field is omitted from the JSON).
+func toHistoryLastMessage(m *history.MessageSummary) *historyLastMessage {
+	if m == nil {
+		return nil
+	}
+	return &historyLastMessage{Role: m.Role, Content: m.Content, CreatedAt: m.CreatedAt}
+}
+
 // handleCreateSubSession creates a new sub session and returns its sessionId. It is a
 // user-side write, so it is owner/admin-gated like /api/v1/chat.
 func (s *Server) handleCreateSubSession(w http.ResponseWriter, r *http.Request) {