feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/ent/schema/payment_audit_log.go

@@ -0,0 +1,54 @@
+package schema
+
+import (
+	"time"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema"
+	"entgo.io/ent/schema/field"
+	"entgo.io/ent/schema/index"
+)
+
+// PaymentAuditLog holds the schema definition for the PaymentAuditLog entity.
+//
+// 删除策略：硬删除
+// PaymentAuditLog 使用硬删除而非软删除，原因如下：
+//   - 审计日志本身即为不可变记录，通常只追加不修改
+//   - 如需清理历史日志，直接按时间范围批量删除即可
+//   - 保持表结构简洁，提升插入和查询性能
+type PaymentAuditLog struct {
+	ent.Schema
+}
+
+func (PaymentAuditLog) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "payment_audit_logs"},
+	}
+}
+
+func (PaymentAuditLog) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("order_id").
+			MaxLen(64),
+		field.String("action").
+			MaxLen(50),
+		field.String("detail").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+		field.String("operator").
+			MaxLen(100).
+			Default("system"),
+		field.Time("created_at").
+			Immutable().
+			Default(time.Now).
+			SchemaType(map[string]string{dialect.Postgres: "timestamptz"}),
+	}
+}
+
+func (PaymentAuditLog) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("order_id"),
+	}
+}