I3.1 — Remove MassTransit & RabbitMQ apparatus

[nickna]

Phase 3 · Size M · Risk: med

Scope

• Delete MassTransit packages, the MT-backed adapter, Program.Messaging.cs MT config.
• Remove RabbitMQ compose service / env / health check.
• Verify grep clean.

Acceptance criteria

• Zero MassTransit/RabbitMQ references; build + full suite green.

Dependencies

I2.7 stable.

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Part of #909 — Messaging Migration epic.