fix: resolve golangci-lint issues — remove unused constants and functions, fix gofmt

- Remove unused claudeMax*Tokens constants (Claude Max feature not included)
- Remove unused UsageMapHook type, SetUsageMapHook method, and usageToMap function
- Fix gofmt formatting in channel_service.go, openai_model_mapping_test.go,
  chatcompletions_to_responses.go

backend/internal/service/channel_service.go

@@ -134,7 +134,7 @@ func (r ChannelMappingResult) ToUsageFields(reqModel, upstreamModel string) Chan
 
 const (
 	channelCacheTTL       = 10 * time.Minute
-	channelErrorTTL       = 5 * time.Second  // DB 错误时的短缓存
+	channelErrorTTL       = 5 * time.Second // DB 错误时的短缓存
 	channelCacheDBTimeout = 10 * time.Second
 )