fix(lint): 修复 golangci-lint 报告的代码问题

- errcheck: 修复类型断言未检查返回值的问题
  - pool.go: 添加 sync.Map 类型断言安全检查
  - req_client_pool.go: 添加 sync.Map 类型断言安全检查
  - concurrency_cache_benchmark_test.go: 显式忽略断言返回值
  - gateway_service.go: 显式忽略 WriteString 返回值

- gofmt: 修复代码格式问题
  - redis.go: 注释对齐
  - api_key_repo.go: 结构体字段对齐
  - concurrency_cache.go: 字段对齐
  - http_upstream.go: 注释对齐

- unused: 删除未使用的代码
  - user_repo.go: 删除未使用的 sql 字段
  - usage_service.go: 删除未使用的 calculateStats 函数

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/repository/req_client_pool.go

@@ -35,7 +35,9 @@ var sharedReqClients sync.Map
 func getSharedReqClient(opts reqClientOptions) *req.Client {
 	key := buildReqClientKey(opts)
 	if cached, ok := sharedReqClients.Load(key); ok {
-		return cached.(*req.Client)
+		if c, ok := cached.(*req.Client); ok {
+			return c
+		}
 	}
 
 	client := req.C().SetTimeout(opts.Timeout)
@@ -47,7 +49,10 @@ func getSharedReqClient(opts reqClientOptions) *req.Client {
 	}
 
 	actual, _ := sharedReqClients.LoadOrStore(key, client)
-	return actual.(*req.Client)
+	if c, ok := actual.(*req.Client); ok {
+		return c
+	}
+	return client
 }
 
 func buildReqClientKey(opts reqClientOptions) string {