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/ent/paymentorder/paymentorder.go

@@ -28,6 +28,8 @@ const (
 	FieldPayAmount = "pay_amount"
 	// FieldFeeRate holds the string denoting the fee_rate field in the database.
 	FieldFeeRate = "fee_rate"
+	// FieldBonusAmount holds the string denoting the bonus_amount field in the database.
+	FieldBonusAmount = "bonus_amount"
 	// FieldRechargeCode holds the string denoting the recharge_code field in the database.
 	FieldRechargeCode = "recharge_code"
 	// FieldOutTradeNo holds the string denoting the out_trade_no field in the database.
@@ -115,6 +117,7 @@ var Columns = []string{
 	FieldAmount,
 	FieldPayAmount,
 	FieldFeeRate,
+	FieldBonusAmount,
 	FieldRechargeCode,
 	FieldOutTradeNo,
 	FieldPaymentType,
@@ -166,6 +169,8 @@ var (
 	UserNameValidator func(string) error
 	// DefaultFeeRate holds the default value on creation for the "fee_rate" field.
 	DefaultFeeRate float64
+	// DefaultBonusAmount holds the default value on creation for the "bonus_amount" field.
+	DefaultBonusAmount float64
 	// RechargeCodeValidator is a validator for the "recharge_code" field. It is called by the builders before save.
 	RechargeCodeValidator func(string) error
 	// DefaultOutTradeNo holds the default value on creation for the "out_trade_no" field.
@@ -249,6 +254,11 @@ func ByFeeRate(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldFeeRate, opts...).ToFunc()
 }
 
+// ByBonusAmount orders the results by the bonus_amount field.
+func ByBonusAmount(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldBonusAmount, opts...).ToFunc()
+}
+
 // ByRechargeCode orders the results by the recharge_code field.
 func ByRechargeCode(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldRechargeCode, opts...).ToFunc()

backend/ent/paymentorder/where.go

@@ -90,6 +90,11 @@ func FeeRate(v float64) predicate.PaymentOrder {
 	return predicate.PaymentOrder(sql.FieldEQ(FieldFeeRate, v))
 }
 
+// BonusAmount applies equality check predicate on the "bonus_amount" field. It's identical to BonusAmountEQ.
+func BonusAmount(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldEQ(FieldBonusAmount, v))
+}
+
 // RechargeCode applies equality check predicate on the "recharge_code" field. It's identical to RechargeCodeEQ.
 func RechargeCode(v string) predicate.PaymentOrder {
 	return predicate.PaymentOrder(sql.FieldEQ(FieldRechargeCode, v))
@@ -590,6 +595,46 @@ func FeeRateLTE(v float64) predicate.PaymentOrder {
 	return predicate.PaymentOrder(sql.FieldLTE(FieldFeeRate, v))
 }
 
+// BonusAmountEQ applies the EQ predicate on the "bonus_amount" field.
+func BonusAmountEQ(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldEQ(FieldBonusAmount, v))
+}
+
+// BonusAmountNEQ applies the NEQ predicate on the "bonus_amount" field.
+func BonusAmountNEQ(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldNEQ(FieldBonusAmount, v))
+}
+
+// BonusAmountIn applies the In predicate on the "bonus_amount" field.
+func BonusAmountIn(vs ...float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldIn(FieldBonusAmount, vs...))
+}
+
+// BonusAmountNotIn applies the NotIn predicate on the "bonus_amount" field.
+func BonusAmountNotIn(vs ...float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldNotIn(FieldBonusAmount, vs...))
+}
+
+// BonusAmountGT applies the GT predicate on the "bonus_amount" field.
+func BonusAmountGT(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldGT(FieldBonusAmount, v))
+}
+
+// BonusAmountGTE applies the GTE predicate on the "bonus_amount" field.
+func BonusAmountGTE(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldGTE(FieldBonusAmount, v))
+}
+
+// BonusAmountLT applies the LT predicate on the "bonus_amount" field.
+func BonusAmountLT(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldLT(FieldBonusAmount, v))
+}
+
+// BonusAmountLTE applies the LTE predicate on the "bonus_amount" field.
+func BonusAmountLTE(v float64) predicate.PaymentOrder {
+	return predicate.PaymentOrder(sql.FieldLTE(FieldBonusAmount, v))
+}
+
 // RechargeCodeEQ applies the EQ predicate on the "recharge_code" field.
 func RechargeCodeEQ(v string) predicate.PaymentOrder {
 	return predicate.PaymentOrder(sql.FieldEQ(FieldRechargeCode, v))