feat: store sent notifications to DB

[busehalis-sap]

Summary

Some applications need the notifications stored to DB to do further processing with them. This PR adds optional DB storage for sent notifications in both production and local mode.

When cds.notifications.storeNotifications: true is set, each notification is stored to the database after being sent. In production mode, the ANS-assigned notification ID is used directly. In local mode, a UUID is generated locally. Since a single notification can be sent to multiple recipients, one row per recipient is created.

Changes

• NotificationStorage.cds: CDS model with @PersonalData annotations for Notifications, NotificationProperties and NotificationTargetParameters entities
• StoreNotificationsHandler: @After handler on NotificationProviderService that stores notifications to DB after ANS delivery in production mode
• StoreNotificationsLocalHandler: @After handler on ApplicationService that stores notifications to DB in local mode
• NotificationStorageHelper: shared helper class containing the DB persistence logic used by both handlers
• LocalHandler: sets the sent notifications on the event context so StoreNotificationsLocalHandler can access them
• NotificationServiceConfiguration: reads storeNotifications flag and conditionally registers the appropriate handler based on the current mode
• Integration tests: 4 new tests covering DB storage, properties, target parameters and multiple recipients, and 1 test for local mode
• Sample app: demonstrates the feature with StoredNotifications projection and key bookId field

[lisajulia]

Some small changes, otherwise this is fine :)
Thanks!

[Schmarvinius]

just minor stuff

[Schmarvinius]

think its fine but please await lisas review as well

[lisajulia]

Looks good to me now :)