feat(api): roll back started or failed deployments

[ctxswitch]

Parent

Related to #1

What to build

Implement rollback for started or failed Deployment Requests so recovery is explicitly recorded.

Acceptance criteria

• POST /deployments/{id}/rollback transitions started requests to rolled_back.
• POST /deployments/{id}/rollback transitions failed requests to rolled_back.
• Rollback requires a non-empty note.
• Rollback after succeeded returns a conflict error.
• rolled_back is terminal.
• Tests cover rollback from started, rollback from failed, missing rollback note, rollback after success conflict, and terminal rollback behavior.

Blocked by

• feat(api): start approved deployment requests #13
• feat(api): mark deployments failed #15