[Backend] Quality notifications to multiple BPNs

**As** user,
**I want** to be able to send notifications to multiple BPNs,
**so that** I don't have to create notifications for each BPN individually.

## Hints / Details

Implement concept https://github.com/eclipse-tractusx/traceability-foss/issues/849

Frontend implementation: https://github.com/eclipse-tractusx/traceability-foss/issues/900

## Acceptance Criteria

- [ ] Input of multiple BPNs is possible, when creating notifications (should be done first as a separate PR -> to unblock frontend story #900 )
- [ ] During the creation of notifications, a selection of multiple BPNs will lead to the creation of 'duplicate' notifications for each BPN according to the concept
- [ ] Notifications can only be sent between two BPNs (sender and receiver)
- [ ] A case label is added to the data model
- [ ] (if necessary) APIs are adjusted to make filtering/searching for case label possible
- [ ] Update User Manual / Admin manual / Arc42
- [ ] Update tests

## Out of Scope

- Creation of a 'parent' object for 'duplicate' notifications

Start by reading the linked concept in issue #849, then review the backend notification creation flow and its data model. Check the existing API and test coverage before addressing multiple BPN input, duplicate notifications, sender/receiver constraints, case labels, and manual or Arc42 updates. Done means all acceptance criteria are covered and the frontend issue #900 is unblocked.

Written by the indexing model from the issue text.