feat: 品牌重命名 Sub2API -> TianShuAPI

- 前端: 所有界面显示、i18n 文本、组件中的品牌名称
- 后端: 服务层、设置默认值、邮件模板、安装向导
- 数据库: 迁移脚本注释
- 保持功能完全一致，仅更改品牌名称

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/service/redeem_code.go

@@ -1,41 +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
-}
+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
+}