feat(saas): add webhook event notification system (@unplugged)

Webhook infrastructure for external event notifications:
- SQL migration: webhook_subscriptions + webhook_deliveries tables
- Types: CreateWebhookRequest, UpdateWebhookRequest, WebhookDelivery
- Service: CRUD operations + trigger_webhooks + HMAC-SHA256 signing
- Handlers: REST API endpoints (CRUD + delivery logs)
- Worker: WebhookDeliveryWorker with exponential retry (max 3)

NOT YET INTEGRATED: needs mod registration in lib.rs + workers/mod.rs,
hmac crate dependency, and route mounting. Code is ready for future
integration after stabilization phase completes.

crates/zclaw-saas/src/webhook/mod.rs

@@ -0,0 +1,18 @@
+//! Webhook 模块 — 事件通知系统
+//!
+//! 允许 SaaS 平台在事件发生时向外部 URL 发送 HTTP POST 通知。
+//! 支持 HMAC-SHA256 签名验证、投递日志和自动重试。
+
+pub mod types;
+pub mod service;
+pub mod handlers;
+
+use crate::state::AppState;
+
+/// Webhook 路由 (需要认证)
+pub fn routes() -> axum::Router<AppState> {
+    axum::Router::new()
+        .route("/api/v1/webhooks", axum::routing::get(handlers::list_subscriptions).post(handlers::create_subscription))
+        .route("/api/v1/webhooks/:id", axum::routing::patch(handlers::update_subscription).delete(handlers::delete_subscription))
+        .route("/api/v1/webhooks/:id/deliveries", axum::routing::get(handlers::list_deliveries))
+}