The base assets and world.pmtiles were only fetched on a visit to /maps or
/settings/maps. Easy setup downloads map collections without opening
either page, so a box that went offline afterwards had regional extracts
but a grey map everywhere else (#1030).
downloadCollection() now starts ensureBaseAssets() in the background
whenever it queues a download. It isn't awaited: a failure is logged and
the lazy path on /maps and the Download Base Map action still apply.
The in-flight guards for the tarball download and the world extract are
now static. Controllers get a fresh MapService per request, so the
per-instance guard never deduplicated across requests, and easy setup
sends one download-collection request per selected collection.
Fixes#1030
d28c2c9c0c