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_test.go

@@ -223,7 +223,7 @@ func (s *GitHubReleaseServiceSuite) TestFetchLatestRelease_Success() {
 	s.srv = newLocalTestServer(s.T(), http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		require.Equal(s.T(), "/repos/test/repo/releases/latest", r.URL.Path)
 		require.Equal(s.T(), "application/vnd.github.v3+json", r.Header.Get("Accept"))
-		require.Equal(s.T(), "Sub2API-Updater", r.Header.Get("User-Agent"))
+		require.Equal(s.T(), "TianShuAPI-Updater", r.Header.Get("User-Agent"))
 		w.Header().Set("Content-Type", "application/json")
 		w.WriteHeader(http.StatusOK)
 		_, _ = w.Write([]byte(releaseJSON))