HTTP service rendering wallpapers on demand for a location, variant,
overlay and size. Renders stay fresh for 10 minutes; ones that were
picked up are re-rendered in the background when they go stale.
realwallpaper gains --cache-dir so locations share one data cache, and
its cache cleanup now tracks last use instead of fetch time.

Replaces pkgs.realwallpaper (xplanet, dead NASA NEO sources) with a
Python renderer: oblique cylindrical map centred on home with a zoom
lens (or plain/mercator), multi-resolution WMS imagery (Sentinel-2
cloudless, Blue Marble, Black Marble lights), live EUMETSAT clouds with
synthesised detail, VIIRS snow/fire/chlorophyll, sea ice and the sky.

The krebs.realwallpaper module, its nginx config and the mors
fetchWallpaper config are dropped; the tool runs standalone for now.

flake.lock: update