fix: mount postgres 18 data volume at /var/lib/postgresql

The postgres:18 image moved its data directory from
/var/lib/postgresql/data to /var/lib/postgresql, and its entrypoint
now fails hard when a volume is mounted at the old path. This made
paperless-ngx-db restart-loop and never reach healthy, so the deploy
errored with 'dependency paperless-ngx-db failed to start'.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

blueprints/paperless-ngx/docker-compose.yml

@@ -35,7 +35,7 @@ services:
       POSTGRES_USER: paperless
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
     volumes:
-      - paperless-db:/var/lib/postgresql/data
+      - paperless-db:/var/lib/postgresql
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U paperless -d paperless"]
       interval: 10s