Job Scheduling: One-Off Execution

[tmi]

Extend the backend to support one-off execution in the future. API-wise similar to scheduling

Multiple options:

• extend the existing jobs with a delay column/table, and a new status "delayed"
• extend the existing schedules with "type: once", and allow concrete datetime instead of cron expr

The latter is less work, but maybe bit odd on the UI level? Needs refinement/discussion