feat(deployment): record deployment rollback

[ctxswitch]

Parent

Related to #2

What to build

Add the end-to-end rollback path for Deployments that have started, succeeded, or failed. Operators should be able to record rollback as an explicit recovery action with required context, terminal state behavior, current-state updates, and append-only event history.

Acceptance criteria

• POST /deployments/{id}/rollback rolls back Deployments in started status.
• POST /deployments/{id}/rollback rolls back Deployments in succeeded status.
• POST /deployments/{id}/rollback rolls back Deployments in failed status.
• Rollback requests require an explicit actor because authentication is out of scope.
• Rollback requires a non-empty note.
• Rollback records rolled_back_at and updates current status to rolled_back.
• Rollback attempts for pending_approval, approved, rejected, or already rolled_back Deployments return conflict errors distinct from validation errors.
• rolled_back is terminal and cannot be mutated by later review, start, completion, or rollback actions.
• The rollback action appends exactly one deployment_rolled_back event and updates current Deployment state in one SQLite transaction.
• GET /deployments/{id} and GET /deployments/{id}/events reflect rollback state and event history oldest first.
• Tests cover rollback from started/succeeded/failed, terminal-state behavior, required rollback notes, SQLite transaction behavior, HTTP success/conflict responses, event ordering, and make test passes.

Blocked by

• feat(deployment): complete started deployments #7