feat(apiserver): expose PEND reason end-to-end (scheduler → API)

The pending_reason field was declared on models.JobInfo but never populated:
scheduler computed PEND reasons and the controller stored them on
job->pend_reasons, yet the data stopped there. Two gaps closed:

1. controller/apiserver.c: apiserver_job_hook_fun now handles
   HOOK_DETAIL_SETPREASON by pushing the updated job to API servers (reusing
   apiserver_job_update_fun). Without this, a job whose PEND reasons change
   never reached the API server — only JOB_NEW/UPDATE/START/EXIT events did,
   and setpreason fired none of those.

2. backend/convert.go + convert_pure.go: cJobToModel / rawJobToModel now
   extract xce_job_t.pend_reasons (main_reason + host/res reasons) and
   marshal them into models.JobInfo.PendingReason as JSONB. models/jobs.go:
   pending_reason added to JOB_DEFAULT_COLUMN so default job queries return
   it (it was only in JSONBJFields, never selected).

Verified live on t1/t2/t3: a job with resreq "type == nosuchhost" stays
PEND and returns
  "pending_reason":{"main_reason":0,"host_reasons":[{"host":"t1","reason":3},
   {"host":"t2","reason":3},{"host":"t3","reason":3}]}
(main_reason=3 PREASON_MATCH_FAIL per host) via both /jobs list, /jobs/:id
and the SSE stream. 4 new unit tests (rawJobToModel pendReason marshal +
name mapping), all apiserver backend tests pass.

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

src/frontend/apiserver/backend/convert_pure.go

@@ -159,6 +159,47 @@ type rawSubmitData struct {
 	startInfo   *rawStartInfo
 }
 
+// rawHostReason mirrors the C xce_host_reason_t (one host-level PEND reason).
+type rawHostReason struct {
+	Host   string `json:"host"`
+	Reason int    `json:"reason"`
+}
+
+// rawResReason mirrors the C xce_res_reason_t (one resource-level PEND reason).
+type rawResReason struct {
+	Resource string `json:"resource"`
+	Host     string `json:"host"`
+	Reason   int    `json:"reason"`
+}
+
+// rawPendReason mirrors the C xce_job_preason_t attached to a PEND job by the
+// controller (scheduler → setpreason → job->pend_reasons). JSON-tagged so the
+// marshaled value lands in models.JobInfo.PendingReason as a JSONB object —
+// processJobData's JSONBJFields pass then surfaces it in the API response.
+type rawPendReason struct {
+	MainReason   int             `json:"main_reason"`
+	MainReasonName string        `json:"main_reason_name"`
+	HostReasons  []rawHostReason `json:"host_reasons,omitempty"`
+	ResReasons   []rawResReason  `json:"res_reasons,omitempty"`
+}
+
+// pendReasonName maps a PREASON_* main_reason code to a readable name. The
+// codes are the PEND reason bitfield of xce_job_preason_t (PREASON_BAD_RESREQ/
+// NO_RESOURCE/MATCH_FAIL, src/include/preason.h). Unknown codes are reported
+// verbatim so a new scheduler reason never silently disappears.
+func pendReasonName(code int) string {
+	switch code {
+	case 1: // PREASON_BAD_RESREQ
+		return "bad_resreq"
+	case 2: // PREASON_NO_RESOURCE
+		return "no_resource"
+	case 3: // PREASON_MATCH_FAIL
+		return "match_fail"
+	default:
+		return fmt.Sprintf("unknown(%d)", code)
+	}
+}
+
 // rawJobData mirrors the C xce_job_t fields used by the Go model.
 type rawJobData struct {
 	jobID      int64
@@ -170,6 +211,7 @@ type rawJobData struct {
 	finishTime int64
 	submit     *rawSubmitData
 	execHosts  []string
+	pendReason *rawPendReason
 }
 
 // rawWorkerData mirrors the C xce_worker_t fields used by the Go model.
@@ -258,6 +300,11 @@ func rawJobToModel(r rawJobData) *models.JobInfo {
 		data, _ := json.Marshal(execHostsJSON{HostSlots: slots, NumHosts: len(slots)})
 		info.ExecHosts = models.Jsonb{String: string(data)}
 	}
+	if r.pendReason != nil {
+		if data, err := json.Marshal(r.pendReason); err == nil {
+			info.PendingReason = models.Jsonb{String: string(data)}
+		}
+	}
 	return info
 }