fix: address code review findings before merge

- Warn on startup if deprecated STIRLING_VARIANT env var is set
- Broaden upscale.py exception handling to catch RuntimeError/OSError
  for Lanczos fallback (not just ImportError)
- Add QEMU + multi-arch (amd64+arm64) to CI Docker build test
- Use .get() instead of .all() for single-row health check query
- Restore container_name in docker-compose.yml for backwards compat

docker/docker-compose.yml

@@ -4,6 +4,7 @@ services:
       context: ..
       dockerfile: docker/Dockerfile
     image: stirlingimage/stirling-image:latest
+    container_name: stirling-image
     ports:
       - "1349:1349"
     volumes: