feat(manifest): scheduled jobs #331

Problem

Jobs run only around rollouts (before-rollout). Periodic work (backups, restore tests, cleanup, partition maintenance) has to run as loops inside applications, on every replica unless they coordinate.

Proposal

[[spec.jobs]]
name = "backup"
service = "postgres"
command = ["pgbackrest", "backup"]
run = { schedule = "0 3 * * *" }

• A cron schedule, in UTC.
• At most one run at a time per job, with a timeout; a missed run starts once when the daemon comes back.
• Runs are journal actions with their output, visible in Events, and a failed run notifies like a failed deployment.

Wanted by piquel-fr's sourceorigin-dev/sourceorigin#220 and sourceorigin-dev/sourceorigin#215.

Written with Claude Opus 5.5 in Claude Code.