Create LOCAL_RUNBOOK.md

[bigrosss]

Adds a short guide to starting and checking the CTF exchange locally, with quick troubleshooting hints.

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Note

Adds docs/LOCAL_RUNBOOK.md documenting how to run the exchange locally.

• Startup checklist: dependencies, migrations, and starting the service
• Verifications: GET /health and DB connection logs
• Troubleshooting: port conflicts, DB auth, stale data reset

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