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/session_orchestrator_test.go

@@ -175,7 +175,7 @@ func (f *fakeHistory) AppendFull(_ context.Context, msg history.Message) (histor
 	return msg, nil
 }
 
-func (f *fakeHistory) List(context.Context, string, int64, int) ([]history.Message, error) {
+func (f *fakeHistory) List(context.Context, string, int64, int, string) ([]history.Message, error) {
 	return nil, nil
 }
 
@@ -230,6 +230,38 @@ func TestSubmitPersistsUserAndAssistant(t *testing.T) {
 	}
 }
 
+func TestSubmitPersistsDeviceID(t *testing.T) {
+	t.Parallel()
+	run := newGatedRunner(1)
+	hist := &fakeHistory{}
+	o := NewOrchestrator(OrchestratorOptions{
+		Run:     run.Run,
+		Log:     slog.Default(),
+		History: hist,
+	})
+	run.tickets <- struct{}{}
+	// The HTTP/WS boundary threads the browser deviceID through ctx (design/16 §4.5);
+	// both the user turn and the assistant reply must be tagged with it.
+	ctx := withDeviceID(context.Background(), "web:chrome:a1b2c3")
+	sr, res := o.SubmitSync(ctx, "web", []model.Message{userMsg("你好")}, loop.OwnerEnv())
+	if !sr.Accepted || !sr.Immediate {
+		t.Fatalf("submit = %+v, want Accepted+Immediate", sr)
+	}
+	if res == nil {
+		t.Fatal("sync result is nil")
+	}
+	recs := hist.records()
+	if len(recs) != 2 {
+		t.Fatalf("appends = %d, want 2 (user + assistant)", len(recs))
+	}
+	if recs[0].Role != history.RoleUser || recs[0].Content != "你好" || recs[0].DeviceID != "web:chrome:a1b2c3" {
+		t.Fatalf("user record = %+v, want DeviceID web:chrome:a1b2c3", recs[0])
+	}
+	if recs[1].Role != history.RoleAssistant || recs[1].Content == "" || recs[1].DeviceID != "web:chrome:a1b2c3" {
+		t.Fatalf("assistant record = %+v, want DeviceID web:chrome:a1b2c3", recs[1])
+	}
+}
+
 func TestSubmitPersistedBroadcast(t *testing.T) {
 	t.Parallel()
 	run := newGatedRunner(1)