fix(apiserver): map pidinfo from job start into the job model (interactive_job_url)

The worker packs the interactive session URL (ttyd/XJOB) into the start
report's exdata as {"url": ...}; the controller stores it on
job->job.pidinfo->exdata and the capnp Job wire carries pidinfo (pidinfo@9,
JobPIDInfo.exdata@3) — but apiserver's cJobToModel never mapped pidinfo, so
models.JobInfo.InteractiveJobURL (declared only in the query-field whitelist
JobAdditionalFields) had no producer and interactive_job_url was permanently
empty in API responses. xcecli's waitForInteractiveURL/openBrowser flow could
never fire.

- models.JobInfo: add InteractiveJobURL (json interactive_job_url, matching
  the JobAdditionalFields whitelist name).
- convert_pure.go: rawPidInfo (pids/pgids/exdata) + interactiveURLFromExdata;
  rawJobToModel marshals pids/pgids into PIDInfo (pid_info is a JSONBJFields
  entry, parsed by processJobData) and promotes the exdata URL to
  InteractiveJobURL.
- convert.go: cJobToModel extracts job.pidinfo via pidinfo_pid/pidinfo_pgid
  cgo helpers (CGO cannot index the C arrays directly).

Cluster-verified end to end on t1 (fresh apiserver deployed): tty job
(xcecli -I --tty equivalent, exdata {"interactive":1}) now returns
interactive_job_url=http://<worker>:9000 and a populated pid_info; non-tty
jobs return ""/no pid_info. All daemons healthy, zero errors in window.

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

src/frontend/apiserver/models/jobs.go

@@ -573,6 +573,13 @@ type JobInfo struct {
 	PendingReason     Jsonb   `json:"pending_reason"` //change type,  JSONB as []byte
 	PIDInfo           Jsonb   `json:"pid_info"`       // JSONB as []byte
 	JobRusage         Jsonb   `json:"job_rusage"`     // JSONB as []byte
+	// InteractiveJobURL is the ttyd/XJOB session URL the worker reported at
+	// job start (job_pidinfo_t.exdata's {"url": ...} JSON, extracted by the
+	// backend conversion). Empty for non-interactive jobs and for jobs whose
+	// start report predates the exdata plumbing. InteractiveJobURL in
+	// JobAdditionalFields (the query-field whitelist) must stay in sync with
+	// this name.
+	InteractiveJobURL string `json:"interactive_job_url"`
 }
 
 type JobExecHosts struct {