Keep an article's publish date across a draft round-trip

[ssavutu]

Why

Rui Zhao's "Opposites" article lost its publish date during an editing session on Aug 2. The evidence from the live database is unambiguous:

id	slug	creation_date	pub_date
10025	opposites	2026-07-24 13:00:00	2026-08-03 02:17:00
10024	madison-beer…	2026-07-24 13:00:00	2026-07-24 13:00:00
10026	horoscopes-7-24	2026-07-24 13:00:00	2026-07-24 13:00:00

Of 10,001 published articles, 10,000 have pub_date == creation_date and exactly one does not. Its new date is the exact second of a PATCH /v1/articles/opposites in the activity log.

A normal save of a live article was already safe — articlePatchDateColumns leaves pub_date alone — and there is now an integration test proving it. The hole was the draft round-trip: saving as Draft set pub_date = NULL irrecoverably, so the next publish invented today's date.

What changed

• last_pub_date (new column, added idempotently in EnsureArticlesSchema alongside scheduled_pub_date and archived_at — no migration step). Going to draft parks the live date there; publishing without an explicit date reuses the article's own date, current first then parked, and only falls back to now for something genuinely never published.
• Sections pin like authors in the editor. Checked sections and subsections sort to the top and stay visible through a section search, matching the existing byline behavior — unchecking something you can no longer see is how a section assignment silently gets dropped.
• Integration tests (CMS_TEST_DSN, skipped without it) for the publish-date paths and for the scheduled status filter: on its own, combined with the title search, and confirming it stays invisible to anonymous callers.

Verification

go test ./..., tsc --noEmit, npm run lint (one pre-existing warning), npm run build. Integration tests run against a local MariaDB:

CMS_TEST_DSN='…/cms_patch_test?parseTime=true&multiStatements=true' go test ./internal/handlers/ -run ArticlePatchHTTP -v

Not included

The already-damaged row still needs a one-time fix on Delta; this PR does not touch production data:

UPDATE articles SET pub_date = '2026-07-24 13:00:00'
WHERE slug = 'opposites' AND pub_date = '2026-08-03 02:17:00';

Separately worth considering: the editor never displays the publish date of a published article (the date field only appears in Schedule mode), so an editor flipping to Draft and back has no way to see the date change underneath them.

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