feat: 品牌重命名 Sub2API -> TianShuAPI (基于官方 v0.1.46)

- 基于官方最新代码 upstream/main (93db889)
- 包含所有官方新功能：运维监控、促销码系统、Linux DO OAuth 等
- 仅修改品牌显示名称，保持所有功能完整
- 修改范围：前后端界面文字、i18n、邮件模板、日志输出

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

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

backend/internal/repository/github_release_service.go

@@ -52,7 +52,7 @@ func (c *githubReleaseClient) FetchLatestRelease(ctx context.Context, repo strin
 		return nil, err
 	}
 	req.Header.Set("Accept", "application/vnd.github.v3+json")
-	req.Header.Set("User-Agent", "Sub2API-Updater")
+	req.Header.Set("User-Agent", "TianShuAPI-Updater")
 
 	resp, err := c.httpClient.Do(req)
 	if err != nil {