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/api_key.go

@@ -1,20 +1,20 @@
-package service
-
-import "time"
-
-type ApiKey struct {
-	ID        int64
-	UserID    int64
-	Key       string
-	Name      string
-	GroupID   *int64
-	Status    string
-	CreatedAt time.Time
-	UpdatedAt time.Time
-	User      *User
-	Group     *Group
-}
-
-func (k *ApiKey) IsActive() bool {
-	return k.Status == StatusActive
-}
+package service
+
+import "time"
+
+type ApiKey struct {
+	ID        int64
+	UserID    int64
+	Key       string
+	Name      string
+	GroupID   *int64
+	Status    string
+	CreatedAt time.Time
+	UpdatedAt time.Time
+	User      *User
+	Group     *Group
+}
+
+func (k *ApiKey) IsActive() bool {
+	return k.Status == StatusActive
+}