feat: IM式跨端实时同步——message_persisted 广播 + 前端按id去重追加

- history.Store 新增 AppendFull（返回含 id/createdAt 的完整消息），Append 签名不变
- session_orchestrator 加可空 OnPersisted；persistUserMessages/persistAssistantReply 落库后回调广播
- server.go newOrchestrator 接 OnPersisted → hub.broadcast message_persisted（thinking 剥除）
- 前端 ws.ts 加 message_persisted case（仅 activeSessionId 匹配才处理）；appendOrUpdateMessage 升级为 id 精确upsert+内容/时间归一
- 实测：双 WS，A stream 发消息，B 免刷新收到 user+assistant 各1条（count=2 PASS）
- 红线: 身份仍靠 actor 元数据，不靠提示词；用户/助手消息、本端/异端统一路径

internal/services/gateway/session_orchestrator_test.go

@@ -157,11 +157,22 @@ type fakeHistory struct {
 	appends []history.Message
 }
 
-func (f *fakeHistory) Append(_ context.Context, msg history.Message) error {
+func (f *fakeHistory) Append(ctx context.Context, msg history.Message) error {
+	_, err := f.AppendFull(ctx, msg)
+	return err
+}
+
+func (f *fakeHistory) AppendFull(_ context.Context, msg history.Message) (history.Message, error) {
 	f.mu.Lock()
+	defer f.mu.Unlock()
+	if msg.ID == "" {
+		msg.ID = fmt.Sprintf("id-%d", len(f.appends)+1)
+	}
+	if msg.CreatedAt == 0 {
+		msg.CreatedAt = int64(len(f.appends) + 1)
+	}
 	f.appends = append(f.appends, msg)
-	f.mu.Unlock()
-	return nil
+	return msg, nil
 }
 
 func (f *fakeHistory) List(context.Context, string, int64, int) ([]history.Message, error) {
@@ -219,6 +230,44 @@ func TestSubmitPersistsUserAndAssistant(t *testing.T) {
 	}
 }
 
+func TestSubmitPersistedBroadcast(t *testing.T) {
+	t.Parallel()
+	run := newGatedRunner(1)
+	hist := &fakeHistory{}
+	var mu sync.Mutex
+	var persisted []history.Message
+	o := NewOrchestrator(OrchestratorOptions{
+		Run:     run.Run,
+		Log:     slog.Default(),
+		History: hist,
+		OnPersisted: func(m history.Message) {
+			mu.Lock()
+			persisted = append(persisted, m)
+			mu.Unlock()
+		},
+	})
+	run.tickets <- struct{}{}
+	sr, _ := o.SubmitSync(context.Background(), "web", []model.Message{userMsg("你好")}, loop.OwnerEnv())
+	if !sr.Accepted || !sr.Immediate {
+		t.Fatalf("submit = %+v, want Accepted+Immediate", sr)
+	}
+	mu.Lock()
+	got := append([]history.Message(nil), persisted...)
+	mu.Unlock()
+	if len(got) != 2 {
+		t.Fatalf("persisted = %d, want 2 (user + assistant)", len(got))
+	}
+	if got[0].Role != history.RoleUser || got[0].ID == "" || got[0].SessionID != "web" || got[0].Content != "你好" {
+		t.Fatalf("user persisted = %+v", got[0])
+	}
+	if got[1].Role != history.RoleAssistant || got[1].ID == "" || got[1].SessionID != "web" || got[1].Content == "" {
+		t.Fatalf("assistant persisted = %+v", got[1])
+	}
+	if got[0].ID == got[1].ID {
+		t.Fatalf("user and assistant share id %q", got[0].ID)
+	}
+}
+
 func TestSubmitDoesNotPersistRejectedTurn(t *testing.T) {
 	t.Parallel()
 	hist := &fakeHistory{}