fix(scheduler): skip runs missed while unlicensed, log license expiry once

[caioricciuti]

Pro-gating follow-ups from WORK_LOG §0.1 item 4.

1. Missed slots are skipped on resume. While no Pro license allows it, the scheduler ticks do nothing. Before, the first tick after a license was activated ran every overdue schedule once. Now that tick moves each overdue schedule to its next slot without running it (SetScheduleNextRun, logged as "Skipped schedule run missed while paused"). A schedule a person disabled and re-enabled already recomputed its next run, so that path is unchanged.
2. Editing or creating a schedule no longer rewrites its last run. Both used UpdateScheduleStatus, which set last_run_at to now and cleared last_status and last_error. They now move only next_run_at.
3. License expiry is logged once per state. Workers validate the license every tick, so an expired license wrote a warning on every call. Now it logs once when a license enters grace and once when it is fully expired.

Docs: schedules and license pages say missed runs are skipped.

Checked

make vet test passes, including the updated TestTickFollowsLicense (resume tick: 0 runs, next_run_at in the future, last_run_at unset) and a new TestLogExpiryOnce. Site builds. No schema or dependency change.