fix: stop scheduled jobs after switching to multi-user mode

[marmar9615-cloud]

Scheduled jobs are single-user only, but switching an instance to multi-user mode left them running. Their cron timers stayed registered, enqueueScheduledJob still started runs, and every restart registered them again, while every /scheduled-jobs route answered 401, so the admin could no longer see or stop them.

BackgroundService now checks SystemSettings.isMultiUserMode() in two places, the same way the Telegram bot stops itself in multi-user mode. #bootScheduledJobs registers nothing, and enqueueScheduledJob returns null and removes the job's timer instead of starting a run. The second check is what stops timers registered before the switch. The first keeps a multi-user boot from registering timers and logging Registered N scheduled job(s). The job rows are left as they are, so a later multi-user version of the feature still has them.

A worker that is still running at the moment of the switch is ended the next time that job's timer fires, and its run is marked killed.

The trigger route already treats a null run as skipped, the same as when a run is already in flight, and the cron callback ignores the return value.

I ran the real background worker against a local stub LLM, with the job created through POST /scheduled-jobs/new and the switch made through POST /system/enable-multi-user:

after the switch                          master             this branch
cron timer registered before the switch   run completed      no run
enqueueScheduledJob                       returns a run      returns null, no run
boot() in multi-user mode                 1 timer            0 timers
single-user control                       job runs           job runs

Adding only the boot check is not enough, since the timer that is already registered keeps firing until the next restart. The new test in server/__tests__/utils/BackgroundWorkers/index.test.js checks that enqueueScheduledJob starts no run and drops the timer in multi-user mode, and fails on master. The server suite passes, 66 suites and 1419 tests. Prettier passes on both files, and eslint on server/utils/BackgroundWorkers/index.js (the server config ignores __tests__).

Closes #6655