chore(prod): startup migrations, readiness checks and backend healthcheck

- add backend entrypoint that can run alembic upgrade head on startup
- add RUN_MIGRATIONS_ON_STARTUP setting and compose wiring
- add /health/live and /health/ready endpoints with db+redis checks
- add backend container healthcheck against readiness endpoint
- document readiness and startup migration behavior

docker/backend-entrypoint.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env sh
+set -eu
+
+echo "[entrypoint] starting backend container"
+
+if [ "${RUN_MIGRATIONS_ON_STARTUP:-false}" = "true" ]; then
+  echo "[entrypoint] running alembic migrations"
+  alembic upgrade head
+fi
+
+echo "[entrypoint] launching application"
+exec "$@"