feat(api): start approved deployment requests

[ctxswitch]

Parent

Related to #1

What to build

Implement the start action for approved Deployment Requests so the API can record that execution has begun.

Acceptance criteria

• POST /deployments/{id}/start transitions approved requests to started.
• Start accepts an optional note.
• Starting before approval returns a conflict error.
• Starting a terminal request returns a conflict error.
• Tests cover successful start and invalid start transitions.

Blocked by

• feat(api): approve deployment requests #11