Enforce idempotent delivery in the outbox dispatcher to prevent duplicate alerts

[Meshmulla]

Background

src/outbox/ implements the transactional outbox pattern (see OUTBOX_IMPLEMENTATION.md).
Under at-least-once delivery, a dispatcher crash between "send" and "mark processed" can
re-deliver a message, producing duplicate alerts/webhooks downstream.

Proposed work

• Attach a stable idempotency key to every outbox message.
• Persist a processed-keys ledger (with TTL) and short-circuit re-delivery of already-sent keys.
• Propagate the idempotency key as a header so idempotent consumers can dedupe too.

Acceptance criteria

• Every message carries a deterministic idempotency key
• Re-dispatch of a processed key is a no-op
• Idempotency key is forwarded to downstream consumers (e.g. webhook header)
• Tests simulate a crash between send and commit and assert no duplicate delivery

Relevant files

src/outbox/, src/services/alert.service.outbox.ts, OUTBOX_IMPLEMENTATION.md