Found on the real deploy, not in any test: `/data/fonts` in the running
api container was `root:root` and the non-root `node` user could not
write to it. Every upload would have failed with EACCES at runtime while
the api reported ready.
The api Dockerfile already explains the mechanism for uploads and
plugins — Docker copies an image directory's ownership into a fresh named
volume on first mount — and pre-creates them chowned. #303 added
`CUSTOM_FONTS_DIR` to the ENV but not to that mkdir/chown line.
Adds a CI fence so it cannot recur: every `/data/…` path the api image
defaults to must also appear in the mkdir AND the chown. Verified against
the actual defect — removing `/data/fonts` from the chown makes it fail.
