refactor: 调整项目结构为单向依赖

backend/internal/service/redeem_code.go

@@ -0,0 +1,41 @@
+package service
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"time"
+)
+
+type RedeemCode struct {
+	ID        int64
+	Code      string
+	Type      string
+	Value     float64
+	Status    string
+	UsedBy    *int64
+	UsedAt    *time.Time
+	Notes     string
+	CreatedAt time.Time
+
+	GroupID      *int64
+	ValidityDays int
+
+	User  *User
+	Group *Group
+}
+
+func (r *RedeemCode) IsUsed() bool {
+	return r.Status == StatusUsed
+}
+
+func (r *RedeemCode) CanUse() bool {
+	return r.Status == StatusUnused
+}
+
+func GenerateRedeemCode() (string, error) {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(b), nil
+}