feat(api): standardize deployment error responses

[ctxswitch]

Parent

Related to #1

What to build

Standardize deployment API error responses so clients can distinguish validation failures, missing resources, and invalid lifecycle transitions.

Acceptance criteria

• Validation failures return 400 Bad Request.
• Unknown Deployment Request IDs return 404 Not Found.
• Invalid lifecycle transitions return 409 Conflict.
• Error responses use a consistent JSON body with error, message, and optional details.
• Conflict responses include the current status and attempted transition where applicable.
• Tests cover representative validation, not-found, and conflict responses.

Blocked by

• feat(api): create deployment requests #4