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/model/order.go

@@ -18,7 +18,7 @@ type Order struct {
 	Status         string     `json:"status" gorm:"size:20;default:'pending'"` // pending / paid / failed / refunded / cancelled
 	PaymentMethod  string     `json:"payment_method" gorm:"size:50"`           // paddle / stripe / creem / waffo / alipay / wechat
 	PaymentGateway string     `json:"payment_gateway" gorm:"size:50"`
-	GatewayOrderNo string     `json:"gateway_order_no" gorm:"size:200"`
+	GatewayOrderNo string     `json:"gateway_order_no" gorm:"size:200;index"`
 	PaidAt         *time.Time `json:"paid_at,omitempty"`
 	RefundAt       *time.Time `json:"refund_at,omitempty"`
 	ClientIP       string     `json:"client_ip" gorm:"size:50"`