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.

Gemfile

@@ -6,7 +6,6 @@ gem "puma", "~> 8.0"
 gem "rexml", "~> 3.4"
 gem "oauth2", "~> 2.0"
 gem "rubyzip", "~> 2.4"
-gem "sqlite3", "~> 2.0"
 
 group :development do
   gem "rerun"