feat: Docker + SSH deploy for DFX server migration

[davidleomay]

Summary

• Rewrite Dockerfile: multi-stage ARM64 build, Node 20, npm ci, production-only deps
• Add dfx-api-dev.yaml workflow: build + push dfxswiss/dfx-api:beta, SSH deploy to dfxdev
• Add dfx-api-prd.yaml workflow: build + push dfxswiss/dfx-api:latest, SSH deploy to dfxprd
• Mark existing Azure deploy workflows as legacy (kept alive until cutover)

Prerequisites before merge

• GitHub Secrets set: DEPLOY_DEV_SSH_KEY, DEPLOY_DEV_SSH_KNOWN_HOSTS, DEPLOY_DEV_HOST, DEPLOY_DEV_USER
• GitHub Secrets set: DEPLOY_PRD_SSH_KEY, DEPLOY_PRD_SSH_KNOWN_HOSTS, DEPLOY_PRD_HOST, DEPLOY_PRD_USER
• GitHub Secrets set: DOCKER_USERNAME, DOCKER_PASSWORD (may already exist)
• dfx-api case added to deploy.sh on dfxdev/dfxprd (DFXServer/server PR)

Test plan

• CI builds dfxswiss/dfx-api:beta image on Docker Hub
• Image is ARM64 (docker image inspect --format '{{.Architecture}}')
• SSH deploy step succeeds (after deploy.sh + compose are in place)