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/pkg/googleapi/status.go

@@ -1,24 +1,24 @@
-package googleapi
-
-import "net/http"
-
-// HTTPStatusToGoogleStatus maps HTTP status codes to Google-style error status strings.
-func HTTPStatusToGoogleStatus(status int) string {
-	switch status {
-	case http.StatusBadRequest:
-		return "INVALID_ARGUMENT"
-	case http.StatusUnauthorized:
-		return "UNAUTHENTICATED"
-	case http.StatusForbidden:
-		return "PERMISSION_DENIED"
-	case http.StatusNotFound:
-		return "NOT_FOUND"
-	case http.StatusTooManyRequests:
-		return "RESOURCE_EXHAUSTED"
-	default:
-		if status >= 500 {
-			return "INTERNAL"
-		}
-		return "UNKNOWN"
-	}
-}
+package googleapi
+
+import "net/http"
+
+// HTTPStatusToGoogleStatus maps HTTP status codes to Google-style error status strings.
+func HTTPStatusToGoogleStatus(status int) string {
+	switch status {
+	case http.StatusBadRequest:
+		return "INVALID_ARGUMENT"
+	case http.StatusUnauthorized:
+		return "UNAUTHENTICATED"
+	case http.StatusForbidden:
+		return "PERMISSION_DENIED"
+	case http.StatusNotFound:
+		return "NOT_FOUND"
+	case http.StatusTooManyRequests:
+		return "RESOURCE_EXHAUSTED"
+	default:
+		if status >= 500 {
+			return "INTERNAL"
+		}
+		return "UNKNOWN"
+	}
+}