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/rest.go

@@ -70,10 +70,13 @@ type loginResponse struct {
 // triggers streaming over the WebSocket channel and returns immediately. Image is
 // an optional base64 data: URI or http(s) URL attached for the vision path; the
 // loop routes it through the vision/OCR tool or the main model (design/06 §0).
+// DeviceID is the client-produced identifier of the sending browser/device
+// (design/16 §4.5), e.g. "web:chrome:a1b2c3"; it is optional and persisted as-is.
 type chatRequest struct {
 	SessionID string `json:"sessionId"`
 	Text      string `json:"text"`
 	Image     string `json:"image,omitempty"`
+	DeviceID  string `json:"deviceId,omitempty"`
 	Stream    bool   `json:"stream"`
 }
 
@@ -126,7 +129,7 @@ func (s *Server) handleChat(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if req.Stream {
-		s.publishChatStream(r.Context(), req.SessionID, req.Text, req.Image)
+		s.publishChatStream(r.Context(), req.SessionID, req.Text, req.Image, req.DeviceID)
 		writeJSON(w, http.StatusOK, map[string]any{"stream": true})
 		return
 	}
@@ -141,7 +144,7 @@ func (s *Server) handleChat(w http.ResponseWriter, r *http.Request) {
 	// it binds the full-access env: memory read+write, unchanged behavior.
 	msgs := userChatMessages(req.Text, req.Image)
 	if s.hub.orchestrator != nil {
-		sr, res := s.hub.orchestrator.SubmitSync(r.Context(), req.SessionID, msgs, loop.OwnerEnv())
+		sr, res := s.hub.orchestrator.SubmitSync(withDeviceID(r.Context(), req.DeviceID), req.SessionID, msgs, loop.OwnerEnv())
 		if !sr.Accepted {
 			if sr.PendingFull {
 				writeError(w, apperr.Wrap(apperr.ErrConflict, "chat_busy", "会话正忙，待处理消息已满"))