feat(frontend): add production stage to Dockerfile

Multi-stage Dockerfile with three targets:
- development (dev server on :5173, used by docker-compose.yml)
- build (runs npm run build, produces SvelteKit Node-adapter output)
- production (self-contained node build server on :3000)

Node base pinned to node:20.19.0-alpine3.21 for reproducible CI
builds (Renovate will keep it current).

docker-compose.yml now specifies target: development for the
frontend so dev continues to use the dev-server stage. Without
this, Docker would default to the last stage (production).

Refs #497.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -163,6 +163,7 @@ services:
     build:
       context: ./frontend
       dockerfile: Dockerfile
+      target: development  # Dockerfile is multi-stage; default would be the production stage
     container_name: archive-frontend
     restart: unless-stopped
     depends_on: