Create Notification entity to track sent notifications

[Mrigankkh]

Description
Create an optional Notification entity for auditing or future deduplication. This would record each message sent to a user about a given event.

Tasks

Create Notification entity with fields:

subscriberId

eventId

status (e.g. PENDING, SENT)

sentAt

Create JPA repository

Persist each time a notification is sent

Testing

Unit test entity and repository

Integration test to verify record is created