Add Webhook Delivery module

Parent: #172
Blocked by: #173

Goal

Add an internal API module that can create, persist and retrieve
WebhookDelivery records using the types introduced in #173

This establishes the persistence layer without changing existing webhook
dispatch behaviour.

Proposed solution

Create apps/api/src/modules/WebhookDelivery.ts containing a
WebhookDeliveryModule.

The module should initially support:

• Creating and persisting a WebhookDelivery.
• Retrieving a WebhookDelivery by ID.
• Returning null when a delivery does not exist.

New deliveries should:

• Use a generated ID with the whd_z prefix.
• Reference one Event and one Webhook Endpoint.
• Start with pending status.
• Have delivered_at and claim_until set to null.
• Have an empty attempts array.
• Set next_attempt_at so the initial attempt is eligible for processing.

Add WebhookDeliveries to the collections initialized by
Database.EnsureCollections().

Follow the structure and dependency-injection patterns used by existing API
modules.

Expected files

• apps/api/src/modules/WebhookDelivery.ts
• apps/api/src/__tests__/WebhookDelivery.spec.ts
• apps/api/src/modules/Database.ts

Acceptance criteria

• WebhookDeliveryModule can create and persist a delivery.
• Created deliveries use the whd_z ID prefix.
• Created deliveries contain the expected initial values.
• The module can retrieve a persisted delivery by ID.
• Retrieving an unknown ID returns null.
• The WebhookDeliveries collection is initialized with other known
  collections.
• Focused tests cover creation, retrieval and missing deliveries.
• npx nx test api passes.

Non-goals

• Creating or recording delivery attempts
• Delivery state transitions
• Integration with EventService or WebhookDispatcher
• Retry processing
• Scheduler routes
• Public API endpoints or request schemas