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/customer.go

@@ -11,7 +11,7 @@ type Customer struct {
 	ID        uint           `json:"id" gorm:"primaryKey"`
 	Name      string         `json:"name" gorm:"size:100;not null"`
 	Company   string         `json:"company" gorm:"size:200"`
-	Email     string         `json:"email" gorm:"size:100;index"`
+	Email     string         `json:"email" gorm:"size:100;uniqueIndex"`
 	Phone     string         `json:"phone" gorm:"size:20"`
 	Address   string         `json:"address" gorm:"size:500"`
 	Notes     string         `json:"notes" gorm:"type:text"`