fix(purchase): reject a purchase delay where no fire sweep is scheduled #1588

Since #1580, the fire_scheduled_purchases sweep is on by default. A delayed approval (purchase_delay_hours > 0; the migrated default is 0, so this only applies where someone set a delay) still never executes when no fire sweep runs, and nothing warns the user. That happens when:

• enable_fire_scheduled_purchases_schedule = false;
• scheduled tasks are off on GCP or Azure;
• GKE is deployed from terraform/environments/gcp, which passes no enable_scheduled_tasks to GKE.

Such purchases sit in scheduled with no alert. The 24h age cap (#1549) only refuses them on the next sweep run, if one ever runs. #712 suggested refusing to schedule (or failing loudly) when the deployment has no fire sweep.

Fix options:

• the runtime knows whether the sweep is scheduled (an env var set by each module) and executes immediately or refuses with a clear error when it isn't;
• or the settings API rejects purchase_delay_hours > 0 there.

This is the remaining part of #712.

Found in review of #1580.