feat(api): fetch deployment request details

[ctxswitch]

Parent

Related to #1

What to build

Implement the HTTP path for fetching one Deployment Request so reviewers and on-call engineers can inspect request details.

Acceptance criteria

• GET /deployments/{id} returns one Deployment Request by ID.
• Detail responses include all request fields currently supported by the model.
• Unknown IDs return not found.
• Tests cover successful fetch and not-found behavior through the HTTP API.

Blocked by

• feat(api): create deployment requests #4