I2.7 — Staged cutover behind a config switch + rollback

[nickna]

Phase 2 · Size M · Risk: high

Scope

• Flip the IEventBus binding from MassTransit -> Wolverine via config.
• Roll out staging -> prod; keep MassTransit backend switchable for instant rollback for one release cycle.

Acceptance criteria

• Prod runs on Wolverine; rollback rehearsed; stable for an agreed soak period.

Dependencies

I2.6.

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Part of #909 — Messaging Migration epic.