fix(oven): fix stale Fedora base image — fedac→fedora CDN name mismatch
os-builder.mjs was reading from fedac-base-{manifest,latest}.img but
os-base-build.mjs uploads to fedora-base-{manifest,latest}.img. The /os
endpoint was always fetching the old pre-fix manifest from Feb 27.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>