Convert models.py: 4. Notification System

[xlorepdarkhelm]

Note: This issue depends on #1253 to be completed and its PR merged before it can be started.

Converting models.py to SQLAlchemy 2.0 standards is a complex process, this issue is for updating the following model classes:

• Notification
• NotificationHistory
• ServiceInboundApi
• ServiceCallbackApi

These models require the following steps to be done for them (explained in Convert models.py classes to using SQLAlchemy 2.0 standards #1246):

1. Use of Mapped and Type Hints
2. Relationships
3. Use of Mapped for Collections
4. Constraints and Indexes
5. Enum and Other Columns
6. Class Methods and Properties
7. Sessions

Acceptance Criteria:

• Make the necessary changes to the model fields
• Double check that running the migrations and tests still work as expected with no deviation from the previous code
• Verify that a deploy to staging works with the changes once merged

Security Considerations

As SQLAlchemy 2.0+ is now released, the older 1.4 way is deprecated, and may not recieve security updates, as well as are expected to be completely phased out of the codebase in the relatively near future.

Other than that, there's really nothing. The end goal should be that this changes models.py and doesn't impact anything else.