fix: resolve golangci-lint errors (gofmt alignment, errcheck)

- Fix gofmt: align struct field comments in UsageCache, trim trailing
  whitespace on const comments
- Fix errcheck: use comma-ok on type assertion for singleflight result

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/account_usage_service.go

@@ -98,8 +98,8 @@ type antigravityUsageCache struct {
 
 const (
 	apiCacheTTL             = 3 * time.Minute
-	apiErrorCacheTTL        = 1 * time.Minute            // 负缓存 TTL：429 等错误缓存 1 分钟，防止重试风暴
-	apiQueryMaxJitter       = 800 * time.Millisecond     // 用量查询最大随机延迟，打散并发请求避免反滥用检测
+	apiErrorCacheTTL        = 1 * time.Minute        // 负缓存 TTL：429 等错误缓存 1 分钟
+	apiQueryMaxJitter       = 800 * time.Millisecond // 用量查询最大随机延迟
 	windowStatsCacheTTL     = 1 * time.Minute
 	openAIProbeCacheTTL     = 10 * time.Minute
 	openAICodexProbeVersion = "0.104.0"
@@ -326,7 +326,7 @@ func (s *AccountUsageService) GetUsage(ctx context.Context, accountID int64) (*U
 			if flightErr != nil {
 				return nil, flightErr
 			}
-			apiResp = result.(*ClaudeUsageResponse)
+			apiResp, _ = result.(*ClaudeUsageResponse)
 		}
 
 		// 3. 构建 UsageInfo（每次都重新计算 RemainingSeconds）