feat(gateway): webhook 渠道 InboundHandler 挂载到网关（渠道遗留2）

让 webhook 渠道的入站 HTTP 回调真正进运行时（此前 handler 建了但未挂载，回调不达 runtime）。

- channelbridge：Bridge.WebhookHandlers() map[string]http.Handler——遍历适配器，对暴露
  InboundHandler()/Config() 的（*webhook.Adapter）收集 endpointPath→handler。
- gateway/server.go：Deps 加 Webhooks map[string]http.Handler；Server 存 webhooks；
  Handler() 挂载到 mux（无网关鉴权，各 handler 自带 HMAC 签名校验）。
- gateway/static.go：spa 加 isWebhookRoute——已注册的 webhook 路径交给 API mux（否则被 SPA
  fallback 吞成 index.html，webhook 回调不达）。
- wire.go：gateway.Deps 传 Webhooks: a.channelBridge.WebhookHandlers()（wireChannels 在
  wireFrontend 前执行，bridge 已建好）。

实机验证：POST /webhook 空体→bad_request(400,webhook 解码响应非 SPA)、带 JSON→accepted(200)、
GET /webhook 被 webhook 路由处理。go build/vet/test 全绿。

internal/services/channelbridge/channelbridge.go

@@ -16,6 +16,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/http"
 	"strings"
 	"sync"
 	"time"
@@ -28,6 +29,18 @@ import (
 	"git.yeij.top/AskaEth/Cyrene_v2/internal/user"
 )
 
+// webhookAdapter is the optional capability a channel adapter may expose to
+// source an HTTP inbound callback. *webhook.Adapter implements it via
+// InboundHandler(). The bridge reports these so the gateway can mount them.
+type webhookAdapter interface {
+	// InboundHandler returns the http.Handler the gateway mounts at the adapter's
+	// configured endpoint path to receive external callbacks.
+	InboundHandler() http.Handler
+	// Config returns the adapter config snapshot; the endpoint path is read from
+	// it so the gateway can register the correct route.
+	Config() map[string]any
+}
+
 // Knob constants. Compile-time so no bare magic number appears in the run path
 // (project rule §10).
 const (
@@ -123,6 +136,33 @@ func (b *Bridge) Add(a adapter.PlatformAdapter) {
 	b.adapters[a.Name()] = a
 }
 
+// WebhookHandlers returns the inbound HTTP handlers of every registered adapter
+// that exposes an HTTP callback (e.g. *webhook.Adapter), keyed by the endpoint
+// path from the adapter's config. The gateway mounts these to receive external
+// webhook callbacks into the runtime. A nil map is returned when no adapter
+// provides an inbound handler.
+func (b *Bridge) WebhookHandlers() map[string]http.Handler {
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	out := make(map[string]http.Handler)
+	for _, a := range b.adapters {
+		wh, ok := a.(webhookAdapter)
+		if !ok {
+			continue
+		}
+		cfg := wh.Config()
+		path, _ := cfg["endpointPath"].(string)
+		if path == "" {
+			path = "/webhook"
+		}
+		out[path] = wh.InboundHandler()
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
 // Start connects every configured adapter, installs the inbound handler, and
 // launches the read loops. Adapters without credentials (or whose Connect fails)
 // are logged and left disconnected; they never fail startup. Start blocks only