fix(ads): a failed job says which step failed (gh-#724)

[genwave-radio]

Fixes #724 · STORY-435 (a failed job says which step failed) · PLAN T462–T466.

🐛 What was wrong

job_kind is nulled the moment a job settles, success or failure alike, so by the time job_error is readable nobody knows whether the write or the preview produced it. The wizard's Script and Hear steps both rendered the same job.error, so a failed preview showed up on the Script step too.

✅ What changed

• T462 — db/47-ad-spot-job-failed-kind-migration.sh adds station.ad_spot.job_failed_kind text with CHECK (IS NULL OR IN ('write','preview')); db/06 carries the identical column for fresh installs (gh-db/06 fresh-init mirror is missing station.schedule_special (db/36) #618 lesson), pinned by a text-equality spec. No new table, so setup.sh's migration marker stays at db/46 and its range message reports "db/47 adds no new table" — the documented degrade, not a bug.
• T463 — AdSpot.JobFailedKind; StampJobAsync nulls it, ClearJobAsync sets it to the failing job's kind on failure and null on a clean finish (one SQL statement, reads the old job_kind). Fakes mirror the semantics.
• T464 — AdSpotJobDto.FailedKind on the wire as failedKind; job is null only when none of job_kind, job_error, job_failed_kind is set.
• T465 — JobRunner takes the kind it owns (ScriptStep → write, HearStep → preview) and renders the error only when job.failedKind matches. The wizard-level alert keeps request failures only. A legacy row with an error but no failedKind shows on neither step; the next stamped job clears it.
• Specs: tests/GenWave.Host.Tests/Specs/Story435_FailedJobKind.cs (11 facts, endpoint driven against an ephemeral Postgres) and admin-ui/__specs__/wizard-failed-step.spec.tsx. Red at 2b20a02, green now.

🔌 Wire evidence (T466, dev stack via BUILD=1 ./launch.sh, Kokoro stopped)

New spot for Wire Diner through the wizard, "Render preview" on the Hear step:

GET /api/ads/23 -> 200
{"id":23,"state":"draft","job":{"kind":null,"failedKind":"preview","startedAt":null,"waitingForStation":false,
 "error":"preview: ANNOUNCER line 1 of 1 failed to render (HttpRequestException)"}}

select id, state, job_kind, job_failed_kind, left(job_error,90) from station.ad_spot where job_failed_kind is not null;
 23 | draft |  | preview | preview: ANNOUNCER line 1 of 1 failed to render (HttpRequestException)

Hear step (accessibility snapshot):

- button "Render preview"
- alert: "preview: ANNOUNCER line 1 of 1 failed to render (HttpRequestException)"

Back to the Script step: the script textarea and voice cast render, no alert node. Screenshots of both steps are on the dev box (t466-hear-error.png, t466-script-clean.png) and can be attached on request.

📝 Notes for review

• Existing rows that failed before this migration keep job_error with a null job_failed_kind; the migration does not backfill, so their stale error shows on neither step until the next job runs. Accepted rather than guessed.
• db/47 must land in the v5.8.2 pins PR's upgrade note.

Do not merge without Dean.