v19

Cache-bust OTA manifest and binary fetch

A board could read a stale version.json from a CDN/proxy edge and report "up
to date" on an old version while other units saw the new release. Add a unique
cache-busting query plus no-cache headers to the manifest fetch, and the same
to the firmware.bin URL so a stale image can never be flashed after seeing a
newer version.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>