Per-project schedules and bur backup --due

Let each project set its own cadence, so one cron serves them all.

• Resolve bur.schedule (daily, weekly, hourly, <N>h, or <N>d; default daily) per database in discovery.
• bur backup --due backs up only the databases whose last success is older than their cadence: due = now - last_success >= schedule. A missed run catches up on the next tick, because due is based on last success rather than wall-clock slots.
• bur does not manage cron itself. One external timer or cron entry runs bur backup --due. You point cron at the CLI; you don't configure cron from it.

Acceptance

• --due skips databases that aren't due yet and backs up those that are; a missing label means daily.
• a missed run catches up on the next cron.