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 production mode.

When cds.requires.notifications.storeNotifications: true is set, each notification is stored to the database after successful delivery to ANS. The ANS-assigned notification ID is used directly, no new ID is generated. 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
• NotificationServiceConfiguration: reads storeNotifications flag and conditionally registers the handler
• Integration tests: 4 new tests covering DB storage, properties, target parameters and multiple recipients
• Sample app: demonstrates the feature with StoredNotifications projection and key bookId field