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/cmd/server/main.go

@@ -51,7 +51,7 @@ func main() {
 	flag.Parse()
 
 	if *showVersion {
-		log.Printf("Sub2API %s (commit: %s, built: %s)\n", Version, Commit, Date)
+		log.Printf("TianShuAPI %s (commit: %s, built: %s)\n", Version, Commit, Date)
 		return
 	}
 
@@ -101,7 +101,7 @@ func runSetupServer() {
 	// This allows users to run setup on a different address if needed
 	addr := config.GetServerAddress()
 	log.Printf("Setup wizard available at http://%s", addr)
-	log.Println("Complete the setup wizard to configure Sub2API")
+	log.Println("Complete the setup wizard to configure TianShuAPI")
 
 	if err := r.Run(addr); err != nil {
 		log.Fatalf("Failed to start setup server: %v", err)