feat(domain): enforce lifecycle transition rules

[ctxswitch]

Parent

Related to #1

What to build

Implement lifecycle transition rules for Deployment Status so invalid state changes can be rejected consistently.

Acceptance criteria

• pending_approval can transition to approved or rejected.
• approved can transition to started.
• started can transition to succeeded, failed, or rolled_back.
• failed can transition to rolled_back.
• rejected, succeeded, and rolled_back are terminal statuses.
• Rollback after succeeded is not allowed.
• Tests cover all valid transitions and representative invalid transitions.

Blocked by

• feat(domain): define deployment request model #2