ci(backend): 添加 github actions (#10)

## 变更内容

### CI/CD
- 添加 GitHub Actions 工作流（test + golangci-lint）
- 添加 golangci-lint 配置，启用 errcheck/govet/staticcheck/unused/depguard
- 通过 depguard 强制 service 层不能直接导入 repository

### 错误处理修复
- 修复 CSV 写入、SSE 流式输出、随机数生成等未处理的错误
- GenerateRedeemCode() 现在返回 error

### 资源泄露修复
- 统一使用 defer func() { _ = xxx.Close() }() 模式

### 代码清理
- 移除未使用的常量
- 简化 nil map 检查
- 统一代码格式

backend/internal/service/email_service.go

@@ -133,13 +133,13 @@ func (s *EmailService) sendMailTLS(addr string, auth smtp.Auth, from, to string,
 	if err != nil {
 		return fmt.Errorf("tls dial: %w", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	client, err := smtp.NewClient(conn, host)
 	if err != nil {
 		return fmt.Errorf("new smtp client: %w", err)
 	}
-	defer client.Close()
+	defer func() { _ = client.Close() }()
 
 	if err = client.Auth(auth); err != nil {
 		return fmt.Errorf("smtp auth: %w", err)
@@ -303,13 +303,13 @@ func (s *EmailService) TestSmtpConnectionWithConfig(config *SmtpConfig) error {
 		if err != nil {
 			return fmt.Errorf("tls connection failed: %w", err)
 		}
-		defer conn.Close()
+		defer func() { _ = conn.Close() }()
 
 		client, err := smtp.NewClient(conn, config.Host)
 		if err != nil {
 			return fmt.Errorf("smtp client creation failed: %w", err)
 		}
-		defer client.Close()
+		defer func() { _ = client.Close() }()
 
 		auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
 		if err = client.Auth(auth); err != nil {
@@ -324,7 +324,7 @@ func (s *EmailService) TestSmtpConnectionWithConfig(config *SmtpConfig) error {
 	if err != nil {
 		return fmt.Errorf("smtp connection failed: %w", err)
 	}
-	defer client.Close()
+	defer func() { _ = client.Close() }()
 
 	auth := smtp.PlainAuth("", config.Username, config.Password, config.Host)
 	if err = client.Auth(auth); err != nil {