Michele's second dead end today. With no valid .env the stack built for minutes
and then said only "dependency failed to start: container
ambi-box-rtmp-ingest-1 is unhealthy" - compose reports that a dependency failed
and swallows rtmp-ingest's actual refusal, which names the fix. The message
existed and was unreachable.

RTMP_OWNER_KEY and LOOP_SOURCE_KEY are now declared mandatory in the compose
file (`:?` with the fix in the message) rather than defaulted to the placeholders
committed here. Compose interpolates before it pulls, builds or creates
anything, so the failure is now one second on the user's own terminal instead of
ten minutes and a container log they have not been told to open. Verified both
ways: without .env it prints the sentence and stops; with .env it resolves
unchanged.

That costs build, hygiene and security a throwaway env block, because
`docker buildx bake -f docker-compose.yml` interpolates too and a fresh chec...