refactor(docker): split scriptor-app into data + uploads volumes

[bigin]

The previous setup mounted a single named volume scriptor-app over the entire /var/www/scriptor in both containers. Docker only copy-initialises a named volume from the image once, so subsequent docker compose up -d --build updates rebuilt the image but the running container kept reading the OLD code from the volume — making code updates require docker compose down -v, which also wipes the demo DB and uploads.

This split puts code in the IMAGES (rebuilt fresh on every build) and only mutable state in named volumes:

scriptor-data → /var/www/scriptor/data
scriptor-uploads → /var/www/scriptor/public/uploads

The web service now builds from a tiny new docker/Dockerfile.web that bakes public/ into nginx:alpine — both containers have an identical public/ tree, so SCRIPT_FILENAME paths resolve in both when nginx forwards .php to fastcgi. The web container also mounts scriptor-uploads:ro so it can serve user uploads.

Verified locally on macOS Docker 28.1:

• up -d --build → both containers healthy, smoke matrix passes
• down (no -v) + up → DB bytes + mtime preserved exactly
• simulate code change + up -d --build → new code in container, DB still at original bytes + mtime
• down -v → both volumes removed, fresh seed on next up

Migration on existing deployments: a one-time docker compose down -v is needed (the old scriptor-app volume is no longer referenced and can be docker volume rm'd). After that, code updates flow through cleanly without ever touching state again.