Closes #16971: Add system jobs (#17716)

* Fix check for existing jobs

If a job is to be enqueued once and no specific scheduled time is
specified, any scheduled time of existing jobs will be valid. Only if a
specific scheduled time is specified for 'enqueue_once()' can it be
evaluated.

* Allow system jobs to be registered

A new registry key allows background system jobs to be registered and
automatically scheduled when rqworker starts.

* Test scheduling of system jobs

* Fix plugins scheduled job documentation

The documentation reflected a non-production state of the JobRunner
framework left over from development. Now a more practical example
demonstrates the usage.

* Allow plugins to register system jobs

* Rename system job metadata

To clarify which meta-attributes belong to system jobs, each of them is
now prefixed with 'system_'.

* Add predefined job interval choices

* Remove 'system_enabled' JobRunner attribute

Previously, the 'system_enabled' attribute was used to control whether a
job should run or not. However, this can also be accomplished by
evaluating the job's interval.

* Fix test

* Use a decorator to register system jobs

* Specify interval when registering system job

* Update documentation

---------

Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>

netbox/core/management/commands/rqworker.py

@@ -2,6 +2,8 @@ import logging
 
 from django_rq.management.commands.rqworker import Command as _Command
 
+from netbox.registry import registry
+
 
 DEFAULT_QUEUES = ('high', 'default', 'low')
 
@@ -14,6 +16,15 @@ class Command(_Command):
     of only the 'default' queue).
     """
     def handle(self, *args, **options):
+        # Setup system jobs.
+        for job, kwargs in registry['system_jobs'].items():
+            try:
+                interval = kwargs['interval']
+            except KeyError:
+                raise TypeError("System job must specify an interval (in minutes).")
+            logger.debug(f"Scheduling system job {job.name} (interval={interval})")
+            job.enqueue_once(**kwargs)
+
         # Run the worker with scheduler functionality
         options['with_scheduler'] = True