Add integration to automatically discover and register cron jobs from
plugins that expose a `*.getCronJobs` gateway method.
This fixes the checkins plugin's scheduling - previously it registered
the gateway method but the jobs were never actually added to the cron
service. Now on gateway startup, after plugin services initialize:
1. Scan plugin gateway handlers for *.getCronJobs methods
2. Call each handler to get cron job configurations
3. Register jobs with the CronService (deduplicating by name)
The checkins plugin's member schedules will now fire at the configured
times.
