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/billing_cache_port.go

@@ -1,15 +1,15 @@
-package service
-
-import (
-	"time"
-)
-
-// SubscriptionCacheData represents cached subscription data
-type SubscriptionCacheData struct {
-	Status       string
-	ExpiresAt    time.Time
-	DailyUsage   float64
-	WeeklyUsage  float64
-	MonthlyUsage float64
-	Version      int64
-}
+package service
+
+import (
+	"time"
+)
+
+// SubscriptionCacheData represents cached subscription data
+type SubscriptionCacheData struct {
+	Status       string
+	ExpiresAt    time.Time
+	DailyUsage   float64
+	WeeklyUsage  float64
+	MonthlyUsage float64
+	Version      int64
+}