🔔 飞行侠实现：实例注册 + Webhook 推送

新功能:
- instances 应用：OpenClaw 实例管理
- Instance 模型：实例注册，Agent 列表，Webhook URL
- MeetingInstanceMap：会议 - 实例映射
- Webhook 推送：消息发送时自动通知相关实例

API 端点:
- POST /api/v1/instances/register/ - 实例注册
- POST /api/v1/instances/join-meeting/ - 加入会议
- GET  /api/v1/instances/ - 实例列表
- POST /api/v1/instances/webhook-test/ - Webhook 测试

集成:
- send_message API 自动触发 Webhook 推送
- 支持广播和定向推送

测试:
- test_webhook.py: 完整测试流程

使用场景:
1. 每台 OpenClaw 机器注册实例
2. Agent 加入会议时关联实例
3. 消息发送时推送到对应机器
4. 本机 OpenClaw 收到通知，触发 Agent 响应

backend/meeting_room/urls.py

@@ -4,6 +4,7 @@ from django.views.generic import TemplateView
 from rest_framework.routers import DefaultRouter
 from meetings.views import MeetingViewSet, ParticipantViewSet
 from users.views import LoginView, RegisterView
+from instances.views import InstanceRegisterView, MeetingJoinView, InstanceListView, WebhookNotifyView
 
 router = DefaultRouter()
 router.register(r'meetings', MeetingViewSet, basename='meeting')
@@ -14,6 +15,10 @@ urlpatterns = [
     path("", TemplateView.as_view(template_name="meeting_room.html"), name="home"),
     path("api/v1/auth/login/", LoginView.as_view()),
     path("api/v1/auth/register/", RegisterView.as_view()),
+    path("api/v1/instances/register/", InstanceRegisterView.as_view()),
+    path("api/v1/instances/join-meeting/", MeetingJoinView.as_view()),
+    path("api/v1/instances/", InstanceListView.as_view()),
+    path("api/v1/instances/webhook-test/", WebhookNotifyView.as_view()),
     re_path(r'^api/v1/meetings/(?P<pk>[^/.]+)/generate-minutes/$', MeetingViewSet.as_view({'get': 'minutes'}), name='meeting-minutes'),
     path("api/v1/", include(router.urls)),
 ]