nginx.conf was hard-capped at 5M, causing 413 on uploads larger than 5MB
despite the backend accepting up to MAX_FILE_SIZE_MB (default 50).
Both nginx configs become envsubst templates. NGINX_MAX_BODY_SIZE defaults
to 200M so the backend application limit is always the effective arbiter.
gettext-base added to the single-image apt deps to provide envsubst.
