Add Docker healthchecks to all services

[shotwellj]

What

Add HEALTHCHECK instructions to the Dockerfiles for Gateway, Episode Store, Policy Engine, and OTel Collector so Docker Compose can report service health accurately.

Why

Right now docker ps shows all services as 'Up' even if they're not actually responding. Healthchecks let Docker (and users) know when a service is truly ready.

Where to start

• Add a /health or /readyz endpoint to each service if one doesn't exist
• Add HEALTHCHECK to each Dockerfile
• Update docker-compose.yml with depends_on conditions using service_healthy

This is a great first contribution — small, self-contained, and improves the setup experience for everyone.