feat(backthink): 主动推送 sink 打通——hub.SendProactive + bridge.SendProactive + wire 接线

backthink 决策+路由(P2)就绪但无投递 sink(ActiveClient/IMAdapter 无实现)。补齐:
- gateway/proactive.go: hub.SendProactive 向 owner/admin 连接广播 proactive_message WS事件,
  内容脱敏(去RawData/media/token),返回接收数,count=0→IM兜底;Server.SendProactive 委托
- channelbridge/proactive.go: Bridge.SendProactive 解析 owner 账户(msg.Platform→匹配白名单账号,
  否则首个已配置主人频道),ChannelType=Direct+ChannelID/SenderID=owner id,走 adapter.SendProactive
- backthink/sinks.go: SetActiveSink/SetIMSink 晚注入(New后重建pusher,currentPusher 并发安全)
- wire.go: 在 gw/bridge 建好后 bt.SetActiveSink(gw)/SetIMSink(bridge)
- 投递链: think→parseProactive→ShouldPush→currentPusher.Push→gate→active(active_client)/IM(im_bridge)
  →record(route+送达数)
- 测试: sinks/gateway proactive/channelbridge proactive; 后端 build/vet/test 全绿
- 注: -race 本机 TSan 不可用(环境); 多平台IM按msg.Platform匹配 owner 账号(单sink诚实);
  weixin 24h窗口按 ErrProactiveWindow 记录为 im_bridge 错误不静默

internal/services/channelbridge/channelbridge_test.go

@@ -50,7 +50,10 @@ func (f *fakeAdapter) SendMessage(_ context.Context, resp adapter.UnifiedRespons
 	}
 	return f.sendErr
 }
-func (f *fakeAdapter) SendProactive(context.Context, adapter.UnifiedMessage) error { return nil }
+func (f *fakeAdapter) SendProactive(_ context.Context, msg adapter.UnifiedMessage) error {
+	f.sends = append(f.sends, msg)
+	return f.sendErr
+}
 func (f *fakeAdapter) SetMessageHandler(fn adapter.MessageHandler)                 { f.handler = fn }
 func (f *fakeAdapter) Capabilities() adapter.Capabilities                          { return adapter.DefaultCapabilities() }
 func (f *fakeAdapter) ListChannels(context.Context) ([]adapter.ChannelInfo, error) { return nil, nil }