infra: add persistent uploads volume and configure Caddy to serve static images

- Add named Docker volume `uploads` mounted at /app/public/uploads in app container
- Share same volume with Caddy at /srv/uploads for direct static file serving
- Add Caddy `handle /uploads/*` block so images bypass Next.js (standalone mode does not serve runtime public files)
- Create uploads directory with correct nextjs:nodejs ownership in Dockerfile
- Add mkdir safeguard in entrypoint.sh

app/entrypoint.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 set -e
 
+mkdir -p /app/public/uploads
+
 echo "Waiting for database..."
 until pg_isready -h db -p 5432 -U ecommerce; do
   sleep 2