chore(docker): speed up startup and use env_file
- Add start_interval: 2s to all health checks (checks every 2s during
startup instead of waiting 30s, reduces total startup from ~2min to ~20s)
- Use env_file: .env for litellm, app, and netdata services
- Move app service from main compose to prod-only
- Remove commented-out app section from main compose
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>