feat: add transactional outbox for webhook emission (exactly-once)

[Sulamoney222]

Closes #853

Makes WebhookService event emission exactly-once via the transactional outbox.

• Route outbox event_id through the delivery payload so the receiver webhookEventOrdering can dedupe retries.
• WebhookService.emit(event, data, client) writes the outbox row atomically via the caller PoolClient when provided.
• Added enqueueWebhookOutboxEvents helper in src/db/transaction.ts.
• Boot the existing OutboxDispatcher when OUTBOX_DISPATCHER_ENABLED=true.
• Tests added; lint and typecheck clean on changed files.
  Verification: 35/35 targeted tests pass; pre-existing failures on upstream master are unrelated.

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