fix(dev): give the dev backend image an entrypoint so it auto-migrates

[haksungjang]

Root cause (finding 2)

The install-uat single-role job has been red since 2026-06-07 — I traced it
to the dev backend image, not backup/restore (those steps were skipped, never
reached).

apps/backend/Dockerfile (the DEV image, built by docker-compose.dev.yml and
the install-uat single-role job) had CMD ["uvicorn", ...] but no
ENTRYPOINT, so the container ran uvicorn directly and skipped
docker-entrypoint.sh. Its AUTO_MIGRATE auto-migration therefore never ran:

• alembic_version never created → dev healthcheck /health/ready stuck at 503
• backend permanently unhealthy → worker/beat (depends_on backend: service_healthy) never start → install.sh --no-prompt's up -d fails with
  "backend unhealthy".

(Same class as the up -d deadlock the L1 work surfaced.) The prod image
(Dockerfile.prod:98) wires ENTRYPOINT ["/usr/bin/tini","--","/app/docker-entrypoint.sh"];
the dev image simply wasn't wired for it, even though the compose comment already
promised auto-migration.

Fix (apps/backend/Dockerfile)

ENTRYPOINT ["/app/docker-entrypoint.sh"] + an explicit
COPY --chmod=0755 docker-entrypoint.sh (guarantees the +x bit in the layer). The
script applies alembic upgrade head then exec "$@" runs the unchanged
uvicorn --reload CMD. No tini (not in the slim dev image; uvicorn owns its
own SIGTERM), USER stays root for bind-mount uid parity.

Verified locally (clean volumes, before/after)

• before: no alembic upgrade head log, alembic_version absent,
  /health/ready 503, unhealthy.
• after: entrypoint runs the migration → alembic_version at HEAD (0037), 34
  tables, /health/ready 200, healthy at t+5s; uvicorn --reload still
  hot-reloads on a source edit.

Full literal install.sh --no-prompt couldn't run locally (host ports 5432/6379
held by the user's ssh tunnels), so the CI install-uat single-role dispatch is
the authoritative end-to-end confirmation — I'll dispatch it on this branch.

Only apps/backend/Dockerfile + a docker-compose.dev.yml comment changed; prod
image, app code, and install.sh untouched.