feat(backend): opt-in beat split and migration skip in entry.sh

[aatchison]

Legacy ECS behaviour is unchanged by default. The beat split and the migration skip are opt-in flags that appointment-deploy sets.

• CONTAINER_ROLE: api (default), worker, beat, flower. Any other value exits 1.
• CELERY_EMBED_BEAT=false: worker runs without --beat. Unset keeps main's argv.
• SKIP_DB_MIGRATIONS=true: api skips update-db. A failed update-db now exits 1.
• Every role execs its final process, so SIGTERM reaches uvicorn or celery on ECS too.
• Unset CONTAINER_ROLE now means api instead of exit 1. No pulumi path leaves it unset.
• docker-compose.yml: celery-worker sets CELERY_EMBED_BEAT=false; new celery-beat service.

Tested on the built image: legacy worker argv matches main, split worker and standalone beat start, RedBeat lock holds with two beats, SIGTERM gives a warm shutdown.

Part of https://github.com/thunderbird/platform-infrastructure/issues/989 (Appointment stage cutover)
Refs #1077 (migrations move to a one-shot Job in Kubernetes via SKIP_DB_MIGRATIONS; legacy ECS still runs them on entry)
Refs https://github.com/thunderbird/appointment-deploy/issues/19
Refs https://github.com/thunderbird/appointment-deploy/issues/20

[davinotdavid]

lgtm!

QA Log

• Code diff
• Manually ran containers locally