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_handler_test.go

@@ -133,6 +133,41 @@ func TestHistoryListMessagesPagination(t *testing.T) {
 	}
 }
 
+func TestHistoryListMessagesPlatformFilter(t *testing.T) {
+	t.Parallel()
+	_, ts, jwt, st := newHistoryEnv(t)
+	ctx := context.Background()
+	if err := st.Append(ctx, history.Message{SessionID: history.MainSessionID, Role: history.RoleUser, Content: "web", Platform: history.PlatformWeb, CreatedAt: 1000}); err != nil {
+		t.Fatalf("seed web: %v", err)
+	}
+	if err := st.Append(ctx, history.Message{SessionID: history.MainSessionID, Role: history.RoleUser, Content: "tg", Platform: "telegram", CreatedAt: 2000}); err != nil {
+		t.Fatalf("seed tg: %v", err)
+	}
+	tok := signAny(t, jwt, "acct", auth.RoleOwner)
+
+	// No platform → the full main session (both lanes).
+	resp, body := doReq(t, http.MethodGet, ts.URL+"/api/v1/sessions/"+history.MainSessionID+"/messages?limit=10", nil, tok)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("all status = %d, want 200 (body=%s)", resp.StatusCode, body)
+	}
+	var all []historyMessage
+	decodeDataSlice(t, body, &all)
+	if len(all) != 2 {
+		t.Fatalf("all = %d, want 2", len(all))
+	}
+
+	// ?platform=telegram → only the telegram lane, and the DTO carries platform.
+	resp, body = doReq(t, http.MethodGet, ts.URL+"/api/v1/sessions/"+history.MainSessionID+"/messages?limit=10&platform=telegram", nil, tok)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("platform status = %d, want 200 (body=%s)", resp.StatusCode, body)
+	}
+	var tg []historyMessage
+	decodeDataSlice(t, body, &tg)
+	if len(tg) != 1 || tg[0].Content != "tg" || tg[0].Platform != "telegram" {
+		t.Fatalf("telegram page = %+v, want one platform=telegram message", tg)
+	}
+}
+
 func TestHistoryDeleteSubSession(t *testing.T) {
 	t.Parallel()
 	_, ts, jwt, st := newHistoryEnv(t)