实现支付商户池与微信授权配置

新增收款商户、商户池轮询、微信授权配置独立管理；三类新支付
（C端套餐购买、C端资产钱包充值、代理在线预存款充值）无条件
经商户池选择并冻结路由，无旧综合配置回退。merchant_id 为空
历史支付继续按 payment_config_id 双读。凭证版本化加载与
ID+版本缓存保证轮换一致性。删除商户池新支付创建开关及全部
引用。

internal/model/payment_merchant.go

@@ -0,0 +1,94 @@
+package model
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+const (
+	PaymentMerchantStatusDisabled = 0
+	PaymentMerchantStatusEnabled  = 1
+
+	PaymentMerchantStrategyAmount = "amount"
+	PaymentMerchantStrategyCount  = "count"
+	PaymentMerchantStrategyTime   = "time"
+)
+
+// PaymentMerchant 是实际收款商户，凭证只保存在受控字段。
+type PaymentMerchant struct {
+	gorm.Model
+	BaseModel         `gorm:"embedded"`
+	Name              string `gorm:"column:name" json:"name"`
+	PaymentMethod     string `gorm:"column:payment_method" json:"payment_method"`
+	ProviderType      string `gorm:"column:provider_type" json:"provider_type"`
+	MerchantIdentity  string `gorm:"column:merchant_identity" json:"merchant_identity"`
+	Credentials       JSONB  `gorm:"column:credentials;type:jsonb" json:"-"`
+	CredentialVersion int64  `gorm:"column:credential_version" json:"credential_version"`
+	Status            int    `gorm:"column:status" json:"status"`
+	Remark            string `gorm:"column:remark" json:"remark"`
+}
+
+func (PaymentMerchant) TableName() string { return "tb_payment_merchant" }
+
+// PaymentMerchantPool 是一种支付方式的商户轮询池。
+type PaymentMerchantPool struct {
+	gorm.Model
+	BaseModel           `gorm:"embedded"`
+	Name                string     `gorm:"column:name" json:"name"`
+	PaymentMethod       string     `gorm:"column:payment_method" json:"payment_method"`
+	Status              int        `gorm:"column:status" json:"status"`
+	Strategy            string     `gorm:"column:strategy" json:"strategy"`
+	ThresholdAmount     *int64     `gorm:"column:threshold_amount" json:"threshold_amount,omitempty"`
+	ThresholdCount      *int64     `gorm:"column:threshold_count" json:"threshold_count,omitempty"`
+	StatisticCycle      *string    `gorm:"column:statistic_cycle" json:"statistic_cycle,omitempty"`
+	TimePeriodValue     *int64     `gorm:"column:time_period_value" json:"time_period_value,omitempty"`
+	TimePeriodUnit      *string    `gorm:"column:time_period_unit" json:"time_period_unit,omitempty"`
+	TimePeriodStartedAt *time.Time `gorm:"column:time_period_started_at" json:"time_period_started_at,omitempty"`
+	RoutingEpoch        int64      `gorm:"column:routing_epoch" json:"routing_epoch"`
+	Remark              string     `gorm:"column:remark" json:"remark"`
+}
+
+func (PaymentMerchantPool) TableName() string { return "tb_payment_merchant_pool" }
+
+// PaymentMerchantPoolMember 是商户池内按顺序参与轮询的商户。
+type PaymentMerchantPoolMember struct {
+	gorm.Model
+	BaseModel  `gorm:"embedded"`
+	PoolID     uint  `gorm:"column:pool_id" json:"pool_id"`
+	MerchantID uint  `gorm:"column:merchant_id" json:"merchant_id"`
+	SortOrder  int64 `gorm:"column:sort_order" json:"sort_order"`
+}
+
+func (PaymentMerchantPoolMember) TableName() string { return "tb_payment_merchant_pool_member" }
+
+// WechatAuthorization 是 C 端微信 OAuth、小程序与支付 AppID 的全局授权配置。
+type WechatAuthorization struct {
+	gorm.Model
+	BaseModel          `gorm:"embedded"`
+	OaAppID            string `gorm:"column:oa_app_id" json:"oa_app_id"`
+	OaAppSecret        string `gorm:"column:oa_app_secret" json:"-"`
+	OaToken            string `gorm:"column:oa_token" json:"-"`
+	OaAesKey           string `gorm:"column:oa_aes_key" json:"-"`
+	OaOAuthRedirectURL string `gorm:"column:oa_oauth_redirect_url" json:"oa_oauth_redirect_url"`
+	MiniappAppID       string `gorm:"column:miniapp_app_id" json:"miniapp_app_id"`
+	MiniappAppSecret   string `gorm:"column:miniapp_app_secret" json:"-"`
+	CredentialVersion  int64  `gorm:"column:credential_version" json:"credential_version"`
+	Status             int    `gorm:"column:status" json:"status"`
+}
+
+func (WechatAuthorization) TableName() string { return "tb_wechat_authorization" }
+
+// PaymentMerchantRoutingSuccess 是支付首次成功的唯一统计事实。
+type PaymentMerchantRoutingSuccess struct {
+	ID           uint      `gorm:"column:id;primaryKey" json:"id"`
+	PaymentID    uint      `gorm:"column:payment_id" json:"payment_id"`
+	MerchantID   uint      `gorm:"column:merchant_id" json:"merchant_id"`
+	PoolID       uint      `gorm:"column:pool_id" json:"pool_id"`
+	RoutingEpoch int64     `gorm:"column:routing_epoch" json:"routing_epoch"`
+	Amount       int64     `gorm:"column:amount" json:"amount"`
+	PaidAt       time.Time `gorm:"column:paid_at" json:"paid_at"`
+	CreatedAt    time.Time `gorm:"column:created_at" json:"created_at"`
+}
+
+func (PaymentMerchantRoutingSuccess) TableName() string { return "tb_payment_merchant_routing_success" }