feat(api): mark deployments failed

[ctxswitch]

Parent

Related to #1

What to build

Implement the failure action for started Deployment Requests so the API can record failed deployment outcomes with an explanation.

Acceptance criteria

• POST /deployments/{id}/fail transitions started requests to failed.
• Failure requires a non-empty note.
• Failing before start returns a conflict error.
• Tests cover successful failure marking, missing failure note, and invalid failure transitions.

Blocked by

• feat(api): start approved deployment requests #13