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/.dockerignore

@@ -0,0 +1,22 @@
+# Deps are reinstalled inside the image via `npm ci`
+node_modules
+npm-debug.log*
+
+# Secrets / local env — mounted at runtime, never baked in
+.env
+.env.*
+!.env.example
+
+# VCS + tooling
+.git
+.gitignore
+.vscode
+.idea
+
+# Tests + coverage (not needed in the runtime image)
+coverage
+docker-compose*.yml
+**/*.test.js
+
+# Docs
+*.md