Add /api/health endpoint and container healthchecks

There's no way to ask the backend if it's alive. The db container has a healthcheck in docker-compose but the app containers don't, so a wedged backend just sits there looking deployed.

• Add GET /api/health returning { ok: true } plus a cheap DB ping
• Add healthcheck blocks for the backend (hit /api/health) and frontend (HEAD /) services in docker-compose.yml
• depends_on for the frontend can then wait on the backend being healthy rather than just started