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/subscription_plan.go

@@ -0,0 +1,77 @@
+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"
+)
+
+// SubscriptionPlan holds the schema definition for the SubscriptionPlan entity.
+//
+// 删除策略：硬删除
+// SubscriptionPlan 使用硬删除而非软删除，原因如下：
+//   - 套餐为管理员维护的商品配置，删除即表示下架移除
+//   - 通过 for_sale 字段控制是否在售，删除仅用于彻底移除
+//   - 已购买的订阅记录保存在 UserSubscription 中，不受套餐删除影响
+type SubscriptionPlan struct {
+	ent.Schema
+}
+
+func (SubscriptionPlan) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "subscription_plans"},
+	}
+}
+
+func (SubscriptionPlan) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int64("group_id"),
+		field.String("name").
+			MaxLen(100).
+			NotEmpty(),
+		field.String("description").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+		field.Float("price").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,2)"}),
+		field.Float("original_price").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,2)"}).
+			Optional().
+			Nillable(),
+		field.Int("validity_days").
+			Default(30),
+		field.String("validity_unit").
+			MaxLen(10).
+			Default("day"),
+		field.String("features").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+		field.String("product_name").
+			MaxLen(100).
+			Default(""),
+		field.Bool("for_sale").
+			Default(true),
+		field.Int("sort_order").
+			Default(0),
+		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 (SubscriptionPlan) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("group_id"),
+		index.Fields("for_sale"),
+	}
+}