feat: add legacy-api compose and apply script for MIG instances

MIG 인스턴스 startup script가 실행하는 vm/legacy-api/apply.sh를 추가한다.
VM SA의 메타데이터 토큰으로 Secret Manager(selfheal-db-password)에서 DB 비밀번호를
받아 env 파일(0600)에 DSN을 쓰고, 고정 태그 이미지(legacy-api:7186ef0)로 compose up.
Alloy는 WP-15에서 추가한다.

vm/legacy-api/apply.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# legacy-api VM (GCP MIG instance). Idempotent: run at first boot by the startup script and again by `make vm-sync`.
+# 1) DB password from Secret Manager via the VM service account (metadata token; no key files, design 6.11)
+# 2) docker compose up with the pinned image tag
+# Alloy (metrics/logs shipping) is added in WP-15.
+set -euo pipefail
+cd "$(dirname "$0")"
+PROJECT="$(curl -sf -H 'Metadata-Flavor: Google' http://metadata.google.internal/computeMetadata/v1/project/project-id)"
+command -v jq >/dev/null || { apt-get update -qq && apt-get install -y -qq jq >/dev/null; }
+TOKEN="$(curl -sf -H 'Metadata-Flavor: Google' 'http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/token' | jq -r .access_token)"
+DB_PASSWORD="$(curl -sf -H "Authorization: Bearer ${TOKEN}" \
+  "https://secretmanager.googleapis.com/v1/projects/${PROJECT}/secrets/selfheal-db-password/versions/latest:access" \
+  | jq -r .payload.data | base64 -d || true)"
+unset TOKEN
+ENV_FILE=/opt/selfheal/legacy-api.env
+umask 077
+{
+  printf 'DB_DSN=postgres://app:%s@10.20.0.20:5432/app?sslmode=disable\n' "${DB_PASSWORD}"
+} > "$ENV_FILE"
+unset DB_PASSWORD
+docker compose --env-file "$ENV_FILE" -f compose.yaml up -d --pull always --remove-orphans
+docker compose -f compose.yaml ps