Voting-engine outbox delivery and recovery

Implement durable, retryable and idempotent delivery of voting-engine broadcasts, notifications and integration effects. Committed commands must recover from process or enqueue failure without resubmission; rolled-back commands must emit no effects. Include operational visibility and a safe production cutover. Source: [Implement voting-engine outbox delivery](https://task.matda.me/tasks/tsk_zdaraph9uw). Complete the project tasks to finish this work. Implementation details, dependencies, acceptance criteria and questions are in the individual tasks.