build(backend): Dockerize for self-hosted deploy + deploy/log docs

Backend deploy target is self-hosted Docker (VPS / Kubernetes / Docker
Engine), not Cloud Run. Add a multi-stage Dockerfile (Node 20, bcrypt
compiled in build stage, non-root runtime), .dockerignore, a staging
docker-compose, and DEPLOY.md covering install, build, migrate, run, and
log mapping/rotation. Pin engines.node>=20. Update deployment.md runbook
and backend/CLAUDE.md infra line off Cloud Run.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/package.json

@@ -4,6 +4,9 @@
   "description": "Halo Bestie backend API",
   "main": "src/server.js",
   "type": "module",
+  "engines": {
+    "node": ">=20"
+  },
   "scripts": {
     "dev": "node --watch src/server.js",
     "start": "node src/server.js",