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/api_key_repo.go

@@ -311,13 +311,13 @@ func groupEntityToService(g *dbent.Group) *service.Group {
 		return nil
 	}
 	return &service.Group{
-		ID:               g.ID,
-		Name:             g.Name,
-		Description:      derefString(g.Description),
-		Platform:         g.Platform,
-		RateMultiplier:   g.RateMultiplier,
-		IsExclusive:      g.IsExclusive,
-		Status:           g.Status,
+		ID:                  g.ID,
+		Name:                g.Name,
+		Description:         derefString(g.Description),
+		Platform:            g.Platform,
+		RateMultiplier:      g.RateMultiplier,
+		IsExclusive:         g.IsExclusive,
+		Status:              g.Status,
 		SubscriptionType:    g.SubscriptionType,
 		DailyLimitUSD:       g.DailyLimitUsd,
 		WeeklyLimitUSD:      g.WeeklyLimitUsd,

backend/internal/repository/concurrency_cache.go

@@ -126,7 +126,7 @@ var (
 )
 
 type concurrencyCache struct {
-	rdb           *redis.Client
+	rdb            *redis.Client
 	slotTTLSeconds int // 槽位过期时间（秒）
 }
 
@@ -137,7 +137,7 @@ func NewConcurrencyCache(rdb *redis.Client, slotTTLMinutes int) service.Concurre
 		slotTTLMinutes = defaultSlotTTLMinutes
 	}
 	return &concurrencyCache{
-		rdb:           rdb,
+		rdb:            rdb,
 		slotTTLSeconds: slotTTLMinutes * 60,
 	}
 }

backend/internal/repository/concurrency_cache_benchmark_test.go

@@ -22,7 +22,7 @@ func BenchmarkAccountConcurrency(b *testing.B) {
 		_ = rdb.Close()
 	}()
 
-	cache := NewConcurrencyCache(rdb, benchSlotTTLMinutes).(*concurrencyCache)
+	cache, _ := NewConcurrencyCache(rdb, benchSlotTTLMinutes).(*concurrencyCache)
 	ctx := context.Background()
 
 	for _, size := range []int{10, 100, 1000} {

backend/internal/repository/http_upstream.go

@@ -572,7 +572,7 @@ func buildUpstreamTransport(settings poolSettings, proxyURL *url.URL) *http.Tran
 // trackedBody 带跟踪功能的响应体包装器
 // 在 Close 时执行回调，用于更新请求计数
 type trackedBody struct {
-	io.ReadCloser        // 原始响应体
+	io.ReadCloser // 原始响应体
 	once          sync.Once
 	onClose       func() // 关闭时的回调函数
 }

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 {

backend/internal/repository/user_repo.go

@@ -16,15 +16,14 @@ import (
 
 type userRepository struct {
 	client *dbent.Client
-	sql    sqlExecutor
 }
 
 func NewUserRepository(client *dbent.Client, sqlDB *sql.DB) service.UserRepository {
 	return newUserRepositoryWithSQL(client, sqlDB)
 }
 
-func newUserRepositoryWithSQL(client *dbent.Client, sqlq sqlExecutor) *userRepository {
-	return &userRepository{client: client, sql: sqlq}
+func newUserRepositoryWithSQL(client *dbent.Client, _ sqlExecutor) *userRepository {
+	return &userRepository{client: client}
 }
 
 func (r *userRepository) Create(ctx context.Context, userIn *service.User) error {