Dealbot logs should use human-friendly ISO8601 for timestamp

[BigLep]

Done Criteria

When looking at logs (e.g., kubectl --context $CONTEXT -n dealbot logs deployment/dealbot --tail=200) I want to see something like "timestamp":"2026-04-02T18:47:29Z" instead of "timestamp":1775081543138.

Why Important

Makes it easier for a human to reason about the logs since they'll know the datetime when looking.

Notes