I2.4 — Enable transactional outbox for critical/financial publishes

[nickna]

Phase 2 · Size M · Risk: med

Scope

• Turn on the Postgres-backed transactional outbox for spend/financial + webhook publishes (closes the fire-and-forget swallow gap from I1.1).
• Idempotency/inbox where needed.

Acceptance criteria

• A crash between business commit and publish no longer loses the event (verified via fault injection).

Dependencies

I2.2; informed by I0.3.

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Part of #909 — Messaging Migration epic.