feat(api): approve deployment requests

[ctxswitch]

Parent

Related to #1

What to build

Implement approval for Deployment Requests so reviewed requests can move from pending review to ready-to-start.

Acceptance criteria

• POST /deployments/{id}/approve transitions pending_approval requests to approved.
• Approval records the review actor.
• Production approval requires non-empty risk and rollback_plan.
• Non-production approval does not require risk or rollback_plan.
• Invalid approval attempts return conflict or validation errors as appropriate.
• Tests cover successful approval, production readiness validation, non-production approval, and invalid approval transitions.

Blocked by

• feat(api): update pre-review deployment metadata #9
• feat(domain): enforce lifecycle transition rules #10