fix: 按评审报告止血支付漏洞并加固授权链路

废除浏览器回跳自助结算，通用 webhook 强制密钥鉴权，Waffo 支持验签与未知事件忽略；重构订单状态机/激活占座/SQLite WAL，并补备份、readyz、限流与前端导出/登录页安全修复。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/dto/commerce.go

@@ -120,8 +120,8 @@ type ProductWithPlansResponse struct {
 
 // PaymentWebhookRequest 支付回调请求 // Logan.Li
 type PaymentWebhookRequest struct {
-	EventType      string  `json:"event_type" binding:"required"`
-	OrderNo        string  `json:"order_no" binding:"required"`
+	EventType      string  `json:"event_type"`
+	OrderNo        string  `json:"order_no"`
 	GatewayOrderNo string  `json:"gateway_order_no"`
 	Status         string  `json:"status" binding:"required"` // paid / failed / refunded
 	Amount         float64 `json:"amount"`