docs(readme): running notes, edge cases, production checklist

[dkrmerve]

Summary

Documentation only. Adds three README sections:

• Running notes – env vars, ports, useful compose commands, troubleshooting
• Edge cases handled – table mapping each handled case to the test that covers it, plus known limits
• Before going to production – checklist (persistence, auth, CORS lockdown, TLS, request limits, graceful shutdown, observability, resource limits, image hygiene, secrets, backups)

Type

• Feature (docs/* → develop)

Checklist

• No code changes, CI still runs the full pipeline

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