Improve container health checks and subprocess lifecycle handling

[DenWin]

Goal

Make container health and shutdown behavior observable and reliable when ConvertX launches external converter processes.

Proposed work

• Identify or add a lightweight application health endpoint.
• Add a container health check with appropriate intervals and timeouts.
• Test SIGTERM handling during idle operation and active conversions.
• Verify that child converter processes are terminated and reaped.
• Determine whether Compose init: true is sufficient.
• If necessary, evaluate a minimal init such as tini.
• Document expected shutdown and health behavior.

Acceptance criteria

Orchestration can distinguish healthy and unhealthy instances, graceful shutdown completes within a defined timeout, and converter subprocesses do not remain orphaned or zombie processes.