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_provider_instance.go

@@ -0,0 +1,72 @@
+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"
+)
+
+// PaymentProviderInstance holds the schema definition for the PaymentProviderInstance entity.
+//
+// 删除策略：硬删除
+// PaymentProviderInstance 使用硬删除而非软删除，原因如下：
+//   - 服务商实例为管理员配置的支付通道，删除即表示废弃
+//   - 通过 enabled 字段控制是否启用，删除仅用于彻底移除
+//   - config 字段存储加密后的密钥信息，删除时应彻底清除
+type PaymentProviderInstance struct {
+	ent.Schema
+}
+
+func (PaymentProviderInstance) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "payment_provider_instances"},
+	}
+}
+
+func (PaymentProviderInstance) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("provider_key").
+			MaxLen(30).
+			NotEmpty(),
+		field.String("name").
+			MaxLen(100).
+			Default(""),
+		field.String("config").
+			SchemaType(map[string]string{dialect.Postgres: "text"}),
+		field.String("supported_types").
+			MaxLen(200).
+			Default(""),
+		field.Bool("enabled").
+			Default(true),
+		field.String("payment_mode").
+			MaxLen(20).
+			Default(""),
+		field.Int("sort_order").
+			Default(0),
+		field.String("limits").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+		field.Bool("refund_enabled").
+			Default(false),
+		field.Time("created_at").
+			Immutable().
+			Default(time.Now).
+			SchemaType(map[string]string{dialect.Postgres: "timestamptz"}),
+		field.Time("updated_at").
+			Default(time.Now).
+			UpdateDefault(time.Now).
+			SchemaType(map[string]string{dialect.Postgres: "timestamptz"}),
+	}
+}
+
+func (PaymentProviderInstance) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("provider_key"),
+		index.Fields("enabled"),
+	}
+}