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/repository/github_release_service.go

@@ -38,7 +38,7 @@ func (c *githubReleaseClient) FetchLatestRelease(ctx context.Context, repo strin
 	if err != nil {
 		return nil, err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		return nil, fmt.Errorf("GitHub API returned %d", resp.StatusCode)
@@ -63,7 +63,7 @@ func (c *githubReleaseClient) DownloadFile(ctx context.Context, url, dest string
 	if err != nil {
 		return err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("download returned %d", resp.StatusCode)
@@ -78,7 +78,7 @@ func (c *githubReleaseClient) DownloadFile(ctx context.Context, url, dest string
 	if err != nil {
 		return err
 	}
-	defer out.Close()
+	defer func() { _ = out.Close() }()
 
 	// SECURITY: Use LimitReader to enforce max download size even if Content-Length is missing/wrong
 	limited := io.LimitReader(resp.Body, maxSize+1)
@@ -89,7 +89,7 @@ func (c *githubReleaseClient) DownloadFile(ctx context.Context, url, dest string
 
 	// Check if we hit the limit (downloaded more than maxSize)
 	if written > maxSize {
-		os.Remove(dest) // Clean up partial file
+		_ = os.Remove(dest) // Clean up partial file (best-effort)
 		return fmt.Errorf("download exceeded maximum size of %d bytes", maxSize)
 	}
 
@@ -106,7 +106,7 @@ func (c *githubReleaseClient) FetchChecksumFile(ctx context.Context, url string)
 	if err != nil {
 		return nil, err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		return nil, fmt.Errorf("HTTP %d", resp.StatusCode)