feat: IM平台列 + deviceID标识 + 会话持久化完善

- agent-96: chat_messages 加 platform 列(迁移0004) + IM主人私聊落主会话web(带platform) + 前端按平台过滤契约 + message_persisted 带 platform(复用 BroadcastPersisted)
- agent-97: chat_messages 加 device_id 列(迁移0005) + history.Message.DeviceID + Web发消息带deviceId(localStorage getClientId) + IM persistToMain 带 deviceID + 前端气泡展示 SourceLabel + 修主会话基础2个bug(fn scanMessage语法/DDL缺列)
- design/16 §4.4(平台) + §4.5(deviceID) 已入档
- 实机: POST /chat 带 deviceId 读回 user/assistant 均带 deviceId+platform; 后端 build/vet/test 绿 + 前端 build 绿
- 红线: 身份靠 actor 元数据; 禁止 shell 读 .env(JWT冒烟由主会话跑)

internal/services/gateway/history.go

@@ -32,6 +32,8 @@ type historyMessage struct {
 	Mood      string `json:"mood,omitempty"`
 	Image     string `json:"image,omitempty"`
 	Thinking  string `json:"thinking,omitempty"`
+	Platform  string `json:"platform,omitempty"`
+	DeviceID  string `json:"deviceId,omitempty"`
 	CreatedAt int64  `json:"createdAt"`
 }
 
@@ -108,8 +110,9 @@ func (s *Server) handleListMessages(w http.ResponseWriter, r *http.Request) {
 		}
 		before = v
 	}
+	platform := r.URL.Query().Get("platform")
 
-	msgs, err := s.history.List(r.Context(), id, before, limit)
+	msgs, err := s.history.List(r.Context(), id, before, limit, platform)
 	if err != nil {
 		s.log.Error("history list messages", "err", err, "session", id)
 		writeError(w, apperr.Wrap(apperr.ErrInternal, "messages_failed", "failed to list messages"))
@@ -157,6 +160,8 @@ func toHistoryMessage(m history.Message) historyMessage {
 		Mood:      m.Mood,
 		Image:     m.Image,
 		Thinking:  m.Thinking,
+		Platform:  m.Platform,
+		DeviceID:  m.DeviceID,
 		CreatedAt: m.CreatedAt,
 	}
 }