Track what was actually built, and flag when it's stale

The .pmtiles files themselves carry no record of what upstream
snapshot they came from — only a local mtime. Each successful build
now writes a small sidecar (offline.pmtiles.meta.json: the Protomaps
build date used; terrain.pmtiles.meta.json: the Mapterhorn source
names/md5sums) recording that, surfaced in the panel next to the
existing size/updated line.

New GET .../check-update routes compare the recorded build against
the live upstream index/manifest — true/false/null, null meaning
"don't know" (no local build yet, or the check itself failed) rather
than a false "no". Kept separate from the plain status endpoints
(which map.js's own topbar dot hits on every page load) since this
one reaches an external service; only called once when the Offline
data panel is actually opened.

Also: flag emoji next to each country in the picker (derived from the
ISO code via Unicode regional indicators, no image assets needed).

docs/development.adoc

@@ -127,6 +127,20 @@ matching `*/build-status` route) — only one of each kind at a time;
 starting a second while one's running gets a 409, as does starting
 either with nothing selected yet.
 
+Each successful build also writes a small sidecar (`offline.pmtiles.
+meta.json` / `terrain.pmtiles.meta.json` — the Protomaps build date used,
+or the Mapterhorn source names/md5sums) recording what was actually
+built, since the `.pmtiles` file itself carries no such record. `GET
+/api/offline-tiles`/`/api/terrain-tiles` include it as `meta`, shown in
+the panel next to the existing size/updated line. `GET .../check-update`
+compares that recorded build against the live upstream index/manifest
+(without rebuilding anything) and reports `update_available: true/false/
+null` — `null` means "don't know" (no local build yet, or the upstream
+check itself failed), kept as its own route rather than folded into the
+plain status endpoints above since those are on the page-load hot path
+(the topbar's offline-status dot) and this one reaches an external
+service, only worth doing when the Offline data panel is actually open.
+
 == Camper-relevant POIs (Overpass)
 
 `rake overpass:pois[south,west,north,east,out.geojson]` queries