feat(api): mark deployments succeeded

[ctxswitch]

Parent

Related to #1

What to build

Implement the success action for started Deployment Requests so the API can record completed successful deployments.

Acceptance criteria

• POST /deployments/{id}/succeed transitions started requests to succeeded.
• Succeed accepts an optional note.
• Succeeding before start returns a conflict error.
• succeeded is terminal.
• Tests cover successful success marking, invalid success transitions, and terminal success behavior.

Blocked by

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