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/concurrency_service.go

@@ -9,12 +9,6 @@ import (
 )
 
 const (
-	// Wait polling interval
-	waitPollInterval = 100 * time.Millisecond
-
-	// Default max wait time
-	defaultMaxWait = 60 * time.Second
-
 	// Default extra wait slots beyond concurrency limit
 	defaultExtraWaitSlots = 20
 )
@@ -31,7 +25,7 @@ func NewConcurrencyService(cache ports.ConcurrencyCache) *ConcurrencyService {
 
 // AcquireResult represents the result of acquiring a concurrency slot
 type AcquireResult struct {
-	Acquired   bool
+	Acquired    bool
 	ReleaseFunc func() // Must be called when done (typically via defer)
 }
 
@@ -54,7 +48,7 @@ func (s *ConcurrencyService) AcquireAccountSlot(ctx context.Context, accountID i
 
 	if acquired {
 		return &AcquireResult{
-			Acquired:    true,
+			Acquired: true,
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()
@@ -90,7 +84,7 @@ func (s *ConcurrencyService) AcquireUserSlot(ctx context.Context, userID int64,
 
 	if acquired {
 		return &AcquireResult{
-			Acquired:    true,
+			Acquired: true,
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()