Oban improvements

• add missing Oban timeouts
• fix some jobs returning :error instead of :cancel (due to disabled features or other conditions)
• move PurgeExpiredActivity to background queue instead of slow (it isn't actually a slow-running job)
• increase background queue concurrency to 20
• create a DeleteWorker for user and instance deletion jobs which will use the slow queue
• increase slow queue concurrency to 5
• minor Oban version bump (mostly unique insert bugfixes)