Add app:infra:snapshot command, page reads from var/infra.json

The /admin/infra page was slow because Docker stats API blocks per container.
Now a cron (every 5min) generates var/infra.json via app:infra:snapshot,
and the page reads the static JSON file instantly.
Mount Docker socket in cron container for snapshot access.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose-dev.yml

@@ -179,8 +179,11 @@ services:
       dockerfile: Dockerfile
     container_name: e_ticket_cron
     restart: unless-stopped
+    group_add:
+      - "${DOCKER_GID:-989}"
     volumes:
       - .:/app
+      - /var/run/docker.sock:/var/run/docker.sock:ro
     depends_on:
       pgbouncer:
         condition: service_healthy