feat: P0-1e expand handleConfigPush with host config and cluster dispatch

Wires the remaining cluster_config_t data into the notification pipeline:
- dispatchConfigPush() extracted as a pure-Go helper (CGO-free, testable)
  that drives OnQueueUpdated, OnResourceUpdated, OnHostConfigUpdated, and
  OnClusterUpdated from a single config push event
- handleConfigPush() delegates to dispatchConfigPush via the new helper,
  eliminating the previous inline loops
- OnHostConfigUpdated and OnClusterUpdated added to NotificationHandler
  interface and implemented with partial-merge semantics in DefaultHandler
- rawHostConfigData/rawHostConfigToModel and rawClusterData/rawClusterToModel
  added to convert_pure.go with their CGO extraction counterparts in convert.go

Safety fixes from code review:
- OnHostUpdated, OnHostConfigUpdated, OnClusterUpdated now copy *incoming to
  a local merged value before mutation, closing the TOCTOU window between
  store.Get and store.Put under concurrent pushes
- cClusterConfQueues and cClusterConfResources gain nil-pointer guards
  matching the existing guard in cClusterConfHostConfigs
- handleConfigPush gains an explicit gHandler nil check
- cClusterConfToClusterInfo returns nil on empty clustername (prevents phantom
  empty-key write to store.Clusters); handleConfigPush logs a warning when nil
- 36 unit tests passing (CGO_ENABLED=0)

src/frontend/apiserver/backend/notification.go

@@ -11,6 +11,8 @@ type NotificationHandler interface {
 	OnJobDeleted(jobid uint64)
 	OnQueueUpdated(queue *models.QueueInfo)
 	OnHostUpdated(host *models.HostInfo)
+	OnHostConfigUpdated(host *models.HostInfo)
+	OnClusterUpdated(cluster *models.ClusterInfo)
 	OnResourceUpdated(res *models.ResourceDefinitionInfo)
 	OnResourceLocationUpdated(res *models.ResourceDistributionInfo)
 }