Build terrain.pmtiles from Mapterhorn instead of fetching tiles ourselves

Mapterhorn (mapterhorn.com, Protomaps-affiliated) publishes pre-built
Terrarium-encoded terrain as PMTiles: a global planet.pmtiles (z0-12)
plus ~450 regional chunks (z13+). Same pmtiles extract --bbox=...
mechanism offline.pmtiles already uses, just against their files —
confirmed live against their real manifest and the documented
Interlaken example. Replaces fetching every individual tile ourselves
(even after this session's earlier concurrency fix, still tens of
minutes for a real region) with 1-3 extract calls plus a merge when
the bbox spans more than one chunk: seconds, not minutes.

Removes CONCURRENT_TILE_FETCHES, write_terrarium_mbtiles, tile_range,
lng/lat_to_tile_x/y, fetch_terrarium_tile, and the sqlite3 gem
entirely — none of it is needed once the tiles arrive pre-packaged.

dem.js's point-elevation lookup (shadow.js's terrain-shadow overlay)
hardcoded a 256px PNG tile assumption that Mapterhorn's 512px WebP
tiles would have silently broken — now reads the actually-decoded
tile's own size and mime type instead of assuming either.

docs/development.adoc

@@ -58,13 +58,28 @@ no API key). For it to work offline too, place a PMTiles archive of
 Terrarium-encoded `raster-dem` tiles at `data/terrain.pmtiles`
 (`GET /api/terrain-tiles` reports `available: true` once it's there,
 same pattern as offline map data above). Also generatable in-app (see
-below) or by hand — build one locally from a DEM source (e.g.
-Copernicus GLO-30 or SRTM) with a tool like
-https://github.com/mapbox/rio-rgbify[rio-rgbify] (GeoTIFF ->
-Terrarium-encoded PNG tiles in an `.mbtiles`), then
-`pmtiles convert out.mbtiles data/terrain.pmtiles` with
-https://github.com/protomaps/go-pmtiles[go-pmtiles], same as
-`data/offline.pmtiles`.
+below) or by hand, the same `go-pmtiles extract` mechanism as
+`data/offline.pmtiles` above — but from
+https://mapterhorn.com[Mapterhorn], a Protomaps-affiliated project
+publishing pre-built Terrarium-encoded terrain as PMTiles (a global
+`planet.pmtiles`, z0-12, plus ~450 regional chunks covering z13 and up
+— see https://mapterhorn.com/data-access/[their own data-access page]
+for the full manifest and per-source license/attribution details, CC-BY-4.0
+and similar, varying by region):
+
+[source,shell]
+----
+pmtiles extract https://download.mapterhorn.com/planet.pmtiles planet-extract.pmtiles --bbox=west,south,east,north --maxzoom=15
+# only if a regional chunk covers the bbox too (see download_urls.json) —
+# most bboxes hit exactly one, a bbox straddling a chunk boundary hits two:
+pmtiles extract https://download.mapterhorn.com/<chunk>.pmtiles chunk-extract.pmtiles --bbox=west,south,east,north --maxzoom=15
+pmtiles merge planet-extract.pmtiles chunk-extract.pmtiles data/terrain.pmtiles
+----
+
+Tiles are 512px WebP, not the 256px PNG this app's *online* Terrarium
+source uses — `public/js/dem.js`'s own point-elevation lookups
+(`shadow.js`'s terrain-shadow overlay) read the actual decoded tile
+size rather than assuming one, so this isn't a problem either way.
 
 == In-app regeneration
 
@@ -78,11 +93,12 @@ wraps exactly the manual `go-pmtiles` workflows documented above
 (`lib/maptool/pmtiles_builder.rb`) — the server needs the
 https://github.com/protomaps/go-pmtiles[go-pmtiles] CLI installed and
 on `PATH` (override with the `PMTILES_BIN` env var if it lives
-somewhere else); regenerating terrain.pmtiles this way fetches
-individual Terrarium tiles itself and packs them into an MBTiles file
-(the `sqlite3` gem) before the same `pmtiles convert` step, since no
-public Terrarium-encoded PMTiles archive exists to `extract` from the
-way offline.pmtiles above can.
+somewhere else). Fetches Mapterhorn's own small JSON manifest to work
+out which regional chunk(s), if any, intersect the bbox, `extract`s
+each (plus `planet.pmtiles`, always), then `merge`s them if there was
+more than one — typically seconds, not the tens of minutes an earlier
+version of this (fetching every individual Terrarium tile itself) used
+to take for a real-sized region.
 
 A build runs in the background (`POST /api/offline-tiles/build` /
 `/api/terrain-tiles/build`, polled via the matching `*/build-status`