feat: 实现注册优惠码功能

- 支持创建/编辑/删除优惠码，设置赠送金额和使用限制
  - 注册页面实时验证优惠码并显示赠送金额
  - 支持 URL 参数自动填充 (?promo=CODE)
  - 添加优惠码验证接口速率限制
  - 使用数据库行锁防止并发超限
  - 新增后台优惠码管理页面，支持复制注册链接

backend/ent/user_create.go

@@ -13,6 +13,7 @@ import (
 	"entgo.io/ent/schema/field"
 	"github.com/Wei-Shaw/sub2api/ent/apikey"
 	"github.com/Wei-Shaw/sub2api/ent/group"
+	"github.com/Wei-Shaw/sub2api/ent/promocodeusage"
 	"github.com/Wei-Shaw/sub2api/ent/redeemcode"
 	"github.com/Wei-Shaw/sub2api/ent/usagelog"
 	"github.com/Wei-Shaw/sub2api/ent/user"
@@ -271,6 +272,21 @@ func (_c *UserCreate) AddAttributeValues(v ...*UserAttributeValue) *UserCreate {
 	return _c.AddAttributeValueIDs(ids...)
 }
 
+// AddPromoCodeUsageIDs adds the "promo_code_usages" edge to the PromoCodeUsage entity by IDs.
+func (_c *UserCreate) AddPromoCodeUsageIDs(ids ...int64) *UserCreate {
+	_c.mutation.AddPromoCodeUsageIDs(ids...)
+	return _c
+}
+
+// AddPromoCodeUsages adds the "promo_code_usages" edges to the PromoCodeUsage entity.
+func (_c *UserCreate) AddPromoCodeUsages(v ...*PromoCodeUsage) *UserCreate {
+	ids := make([]int64, len(v))
+	for i := range v {
+		ids[i] = v[i].ID
+	}
+	return _c.AddPromoCodeUsageIDs(ids...)
+}
+
 // Mutation returns the UserMutation object of the builder.
 func (_c *UserCreate) Mutation() *UserMutation {
 	return _c.mutation
@@ -593,6 +609,22 @@ func (_c *UserCreate) createSpec() (*User, *sqlgraph.CreateSpec) {
 		}
 		_spec.Edges = append(_spec.Edges, edge)
 	}
+	if nodes := _c.mutation.PromoCodeUsagesIDs(); len(nodes) > 0 {
+		edge := &sqlgraph.EdgeSpec{
+			Rel:     sqlgraph.O2M,
+			Inverse: false,
+			Table:   user.PromoCodeUsagesTable,
+			Columns: []string{user.PromoCodeUsagesColumn},
+			Bidi:    false,
+			Target: &sqlgraph.EdgeTarget{
+				IDSpec: sqlgraph.NewFieldSpec(promocodeusage.FieldID, field.TypeInt64),
+			},
+		}
+		for _, k := range nodes {
+			edge.Target.Nodes = append(edge.Target.Nodes, k)
+		}
+		_spec.Edges = append(_spec.Edges, edge)
+	}
 	return _node, _spec
 }