[19.0] Port cron jobrunner and subscribe addons

[dhongu]

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[dhongu]

Closing as superseded. The two addons this PR covered are handled by dedicated per-addon PRs, both further along than this one:

• queue_job_cron_jobrunner → [19.0][MIG] queue_job_cron_jobrunner: migrate #876 (two approvals, plus two reports of it running in production/staging on 19.0)
• queue_job_subscribe → [MIG] queue_job_subscribe: Migration to 19.0 #975 (three approvals, approved + ready to merge)

Both also do the housekeeping this PR left out: removing the addon from the not-installable exclude list in .pre-commit-config.yaml — without which pre-commit keeps silently skipping an addon that just became installable again — and regenerating README/index for the 19.0 branch.

I verified both on a clean 19.0 database before closing: queue_job_cron_jobrunner: 7 tests, queue_job_subscribe: 4 tests, 0 failed and 0 errors in each.