fix: auto-enable cluster bootstrap, add init-db.sql to postgres volumes

- Auto-enable cluster seeding when BOOTSTRAP_CLUSTERS is set
  (no longer requires separate BOOTSTRAP_ENABLE_CLUSTERS=true)
- Add BOOTSTRAP_ENABLE_CLUSTERS to README .env template
- Mount init-db.sql in postgres service volumes

docker-compose.yml

@@ -28,6 +28,7 @@ services:
       - "${POSTGRES_PORT:-15432}:5432"
     volumes:
       - postgres_data:/var/lib/postgresql/data
+      - ./backend/scripts/init-db.sql:/docker-entrypoint-initdb.d/01-init.sql:ro
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-postgres} -d ${POSTGRES_DB:-ocdp}"]
       interval: 10s