feat(previews): expire idle previews #397

Previews are bounded only by previews.max_per_application and previews.max_total. A preview whose branch stays around keeps running, and keeps its deployments and images, until it's pruned by hand (previews prune) or pushed out by a newer one.

An idle expiry would remove previews that haven't deployed in a configurable time, such as previews.idle_days:

• Config and NixOS module: a new daemon setting, off by default.
• Daemon: the existing preview cleanup would also remove expired previews and record an event.
• CLI and dashboard: show when each preview expires.

Found while revalidating retention findings for the cleanup rounds; it is a feature rather than a fix, so it was left out of them.

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