feat(payment): 充值赠送阶梯（按金额档位额外赠送到账额度）

- 后台支付设置新增「充值赠送阶梯」（满 X 送 Y%，RECHARGE_BONUS_TIERS）与 Markdown 活动文案
  （RECHARGE_BONUS_NOTICE）：阈值按用户输入的支付金额命中（取不超过金额的最大档），
  赠送按到账基数（输入 × 充值倍率）计算；严格校验写入、宽松解析读取，订阅订单不参与
- 下单时按当时配置计算赠送并落库：payment_orders 新增 bonus_amount 列（迁移 241），
  amount 仍为到账总额，到账/兑换码/退款逻辑不变；推广返利基数改为 amount - bonus_amount
- checkout-info 下发 recharge_bonus_tiers / recharge_bonus_notice；创建订单与订单查询响应带 bonus_amount
- 充值页：快捷金额按钮右上角「+N%」角标 + 第二行到账金额，金额卡顶部渲染活动文案，
  订单摘要新增赠送额度行并在有赠送时始终显示到账余额；订单列表/详情/支付结果/成功面板显示含赠送
- 后台新增 RechargeBonusTierEditor（行内校验重复/非法阈值，自动排序并显示区间预览）
- 补后端单测、契约测试与前端 vitest，zh/en 文案

backend/internal/handler/payment_handler.go

@@ -149,6 +149,8 @@ func (h *PaymentHandler) GetCheckoutInfo(c *gin.Context) {
 		BalanceRechargeMultiplier:     cfg.BalanceRechargeMultiplier,
 		SubscriptionUSDToCNYRate:      cfg.SubscriptionUSDToCNYRate,
 		RechargeFeeRate:               cfg.RechargeFeeRate,
+		RechargeBonusTiers:            cfg.RechargeBonusTiers,
+		RechargeBonusNotice:           cfg.RechargeBonusNotice,
 		HelpText:                      cfg.HelpText,
 		HelpImageURL:                  cfg.HelpImageURL,
 		StripePublishableKey:          cfg.StripePublishableKey,
@@ -166,6 +168,8 @@ type checkoutInfoResponse struct {
 	BalanceRechargeMultiplier     float64                         `json:"balance_recharge_multiplier"`
 	SubscriptionUSDToCNYRate      float64                         `json:"subscription_usd_to_cny_rate"`
 	RechargeFeeRate               float64                         `json:"recharge_fee_rate"`
+	RechargeBonusTiers            []service.RechargeBonusTier     `json:"recharge_bonus_tiers"`
+	RechargeBonusNotice           string                          `json:"recharge_bonus_notice"`
 	HelpText                      string                          `json:"help_text"`
 	HelpImageURL                  string                          `json:"help_image_url"`
 	StripePublishableKey          string                          `json:"stripe_publishable_key"`
@@ -482,6 +486,7 @@ type PublicOrderResult struct {
 	Amount              float64    `json:"amount"`
 	PayAmount           float64    `json:"pay_amount"`
 	FeeRate             float64    `json:"fee_rate"`
+	BonusAmount         float64    `json:"bonus_amount"`
 	Currency            string     `json:"currency"`
 	PaymentType         string     `json:"payment_type"`
 	OrderType           string     `json:"order_type"`
@@ -517,6 +522,7 @@ func buildPublicOrderResult(order *dbent.PaymentOrder) PublicOrderResult {
 		Amount:              order.Amount,
 		PayAmount:           order.PayAmount,
 		FeeRate:             order.FeeRate,
+		BonusAmount:         order.BonusAmount,
 		Currency:            service.PaymentOrderCurrency(order),
 		PaymentType:         order.PaymentType,
 		OrderType:           order.OrderType,
@@ -625,6 +631,7 @@ type PaymentOrderResult struct {
 	Amount              float64    `json:"amount"`
 	PayAmount           float64    `json:"pay_amount"`
 	FeeRate             float64    `json:"fee_rate"`
+	BonusAmount         float64    `json:"bonus_amount"`
 	Currency            string     `json:"currency"`
 	PaymentType         string     `json:"payment_type"`
 	OutTradeNo          string     `json:"out_trade_no"`
@@ -663,6 +670,7 @@ func sanitizePaymentOrderForResponse(order *dbent.PaymentOrder) *PaymentOrderRes
 		Amount:              order.Amount,
 		PayAmount:           order.PayAmount,
 		FeeRate:             order.FeeRate,
+		BonusAmount:         order.BonusAmount,
 		Currency:            service.PaymentOrderCurrency(order),
 		PaymentType:         order.PaymentType,
 		OutTradeNo:          order.OutTradeNo,