feat(api): update pre-review deployment metadata

[ctxswitch]

Parent

Related to #1

What to build

Allow requesters to update pre-review metadata on a Deployment Request without changing the request identity.

Acceptance criteria

• PATCH /deployments/{id} updates references, risk, and rollback_plan while status is pending_approval.
• service, environment, version, and requested_by remain immutable after creation.
• Successful updates refresh updated_at.
• Unknown IDs return not found.
• Tests cover successful update, immutable-field protection, and not-found behavior.

Blocked by

• feat(api): create deployment requests #4