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/convert_pure.go

@@ -231,3 +231,53 @@ func rawResourceToModel(r rawResourceData) *models.ResourceDefinitionInfo {
 		Interval:     r.interval,
 	}
 }
+
+// rawHostConfigData mirrors the C xce_hostconfig_t fields used by the Go model.
+// Only fields that map to models.HostInfo are included:
+//   - name → HostName, hostmodel → HostModel, hosttype → HostType, mxj → MaxSlot
+//
+// resources (xce_config_t**) and thresholds (xce_res_threshold_t**) have no HostInfo field and are excluded.
+// dispatchwindow (xce_timewindow_config_t*) is deferred — DisptWindow is left as empty string.
+type rawHostConfigData struct {
+	name      string
+	hostmodel string
+	hosttype  string
+	mxj       int
+}
+
+// rawHostConfigToModel maps a rawHostConfigData to a models.HostInfo.
+// Config-sourced fields only; worker-sourced fields remain zero.
+func rawHostConfigToModel(r rawHostConfigData) *models.HostInfo {
+	return &models.HostInfo{
+		HostName:  r.name,
+		HostModel: r.hostmodel,
+		HostType:  r.hosttype,
+		MaxSlot:   r.mxj,
+		// TODO(dispatchwindow): xce_hostconfig_t.dispatchwindow is xce_timewindow_config_t*
+		// (nsegs int + segs[] with open/close time fields). Serializing it requires a Go
+		// mirror struct and json.Marshal. Deferred: no API endpoint currently consumes
+		// DisptWindow. Implement when /xce/v1/hosts returns dispatch window data.
+	}
+}
+
+// rawClusterData mirrors the C cluster_config_t fields used by the Go model.
+// Only config-sourced fields are included (heartbeat_interval, vcpus, inactive_heartbeat_count).
+// Built-in/sourced fields (Version, Status, ControllerName, etc.) come from LoadBuiltinData
+// and are not part of the config push conversion.
+type rawClusterData struct {
+	name                   string
+	heartbeatInterval     int
+	vcpus                 int
+	inactiveHeartbeatCount int
+}
+
+// rawClusterToModel maps a rawClusterData to a models.ClusterInfo.
+// Config-sourced fields only; built-in fields remain zero.
+func rawClusterToModel(r rawClusterData) *models.ClusterInfo {
+	return &models.ClusterInfo{
+		ClusterName:            r.name,
+		HeartbeatInterval:      r.heartbeatInterval,
+		Vcpus:                  r.vcpus,
+		InactiveHeartbeatCount: r.inactiveHeartbeatCount,
+	}
+}