EventBus: publisher confirms

[adedw]

What to build

Publishing is fire-and-forget: ChannelBasicPublishAsync with mandatory: true gives no durability guarantees. Make the publish path await broker confirms (BasicAcksAsync / chained confirms) on top of the connection layer rebuilt in #81, composing with the existing Polly publish-retry.

Acceptance criteria

• Publish awaits a broker ack before completing (or surfaces a nack/timeout as an error).
• Composes with the existing Polly retry policy: unacked publish is retried.
• Confirmation email publish path is verifiably durable in dev (MailPit delivery after publish completes).

Blocked by

• EventBus: auto-reconnect RabbitMQ consumer #81