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/backthink/backthink.go

@@ -157,6 +157,7 @@ type Backthink struct {
 	log       *slog.Logger
 	store     *Store
 	pusher    Pusher
+	gate      PushGate
 	now       func() time.Time
 	sessionID string
 
@@ -187,12 +188,13 @@ func New(cfg Config, deps Deps) (*Backthink, error) {
 		return nil, fmt.Errorf("backthink: new: %w", err)
 	}
 
+	// Resolve the gate up front so the sink setters can rebuild the pusher with it.
+	gate := deps.Gate
+	if gate == nil {
+		gate = newRateGate(cfg.MaxPushWindow, cfg.PushWindow, log)
+	}
 	pusher := deps.Pusher
 	if pusher == nil {
-		gate := deps.Gate
-		if gate == nil {
-			gate = newRateGate(cfg.MaxPushWindow, cfg.PushWindow, log)
-		}
 		pusher = newPusher(deps.Active, deps.IM, gate, log)
 	}
 
@@ -211,6 +213,7 @@ func New(cfg Config, deps Deps) (*Backthink, error) {
 		log:       log,
 		store:     store,
 		pusher:    pusher,
+		gate:      gate,
 		now:       now,
 		sessionID: sessionID,
 	}, nil
@@ -407,7 +410,7 @@ func (b *Backthink) think(parent context.Context, tr Trigger) {
 
 	pushRes := PushResult{Route: RouteNone, Detail: b.interruptReason(push, at, state)}
 	if shouldPush {
-		pushRes = b.pusher.Push(cctx, PushRequest{
+		pushRes = b.currentPusher().Push(cctx, PushRequest{
 			Message:     b.buildProactiveMessage(message, channelHint),
 			ChannelHint: channelHint,
 		})