Groups: store check-ins in GatherPress's check-in flag - #2231

Fixes #2213.

Check-ins now live in GatherPress's own Check_In flag instead of our wporg_groups_checked_in comment meta, so the event page and the wp-admin RSVPs screen show and change the same thing.

• A hook on gatherpress_rsvp_flag_added/_removed keeps our per-date check-in scopes in step, so a wp-admin check-in also switches "attended" over.
• The check-in time is dropped. The flag has no time, and nothing read it.
• Walk-ins without an account stay ours.
• wp wc-groups-check-ins scan|migrate moves existing check-ins. It's a one-off; delete it once run.

Deploy

After deploying, on the events network:

wp --url=events.wordpress.org wc-groups-check-ins scan --verbose
wp --url=events.wordpress.org/group/<slug>/ wc-groups-check-ins migrate --dry-run
wp --url=events.wordpress.org/group/<slug>/ wc-groups-check-ins migrate

Group sites are two path segments deep, so --url needs a --require that returns 2 from site_by_path_segments_count. Until migrate runs on a site, its old check-ins don't show.

Testing

1. On a past event, open Check in attendees and check someone in.
2. wp-admin → Events → RSVPs for that event. Checked in reads "Yes".
3. Bulk-action Check in another RSVP there.
4. Back on the event, the modal shows both checked in, and Attended lists both.
5. Seed wporg_groups_checked_in on an RSVP, run scan, migrate --dry-run, migrate, then migrate again (nothing left).

• phpunit, full: 2862 passing (9 skipped)
• phpcs clean
• Steps 1-5 locally on GatherPress 0.36.0

Screenshots

Description	Screenshot
Checked in from the event page
wp-admin shows it
Checked in from wp-admin
Event page shows the wp-admin check-in

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