feat: add CronJob and CronJobRun models with bulk upsert API and admin
- CronJob: maps jobs.json (schedule, payload, delivery, state fields)
- CronJobRun: stores runs/*.jsonl per-job execution history with usage/tokens
- cron_bulk_upsert service: atomic upsert with GET_OR_CREATE for idempotency
- POST /api/cron/bulk_upsert/ endpoint
- Django Admin: CronJobAdmin with CronJobRunInline, CronJobRunAdmin
- sync_sessions.py --cron mode: SSH read jobs.json + runs/*.jsonl, incremental sync
- 0003_cronjob_cronjobrun migration