SRTM profiles no longer abort on missing tiles — water/ocean areas
return 0m elevation instead of failing the entire profile and falling
back to the rate-limited API. Elevation client and viewshed now pass
download: true to auto-fetch missing tiles from S3. NFS mount changed
to writable so downloaded tiles persist across pods.
When a local .hgt tile is missing, download it from the public AWS S3
skadi bucket, decompress with zlib, and write to the tiles directory
before retrying the lookup. Falls back to Open-Meteo/OpenTopo APIs if
the download fails.
