Fix: Resolve Django 5.x database access during app initialization warning

[ricosalomar]

Fix: Resolve Django 5.x database access during app initialization warning

1. Root Cause & Context

In Django 5.0+, a new RuntimeWarning was introduced to discourage database operations during the application registry population phase (inside AppConfig.ready() or during module imports):

RuntimeWarning: Accessing the database during app initialization is discouraged. To fix this warning, avoid executing queries in AppConfig.ready() or when your app modules are imported.

In django-heralder, the ready() method of HeraldConfig called Notification.objects.get_or_create(...) to automatically discover and register notification classes. This triggered the warning and caused issues such as potential connection deadlocks in some test suites and startup warning noise.

2. Solution (Signal-Based Registration)

This PR implements an idiomatic, safe, and self-contained adjustment inside src/herald/apps.py:

• Deferred Database Access via post_migrate: We connected the notification auto-discovery database registration loop to Django's post_migrate signal instead of running it directly inside ready().
• No Warning Suppression Required: Since database queries are no longer executed during standard startup or application loading, this completely eliminates the RuntimeWarning without silencing other system warnings.
• Test Suite Alignment: Inside tests/test_usernotifications.py, we updated the setUp() method to use get_or_create() instead of .save() when instantiating Notification test records. Since test environments run migrations and dispatch post_migrate signals automatically, the notifications are already cleanly pre-registered in the test DB, avoiding potential uniqueness IntegrityErrors.

These changes are fully backward-compatible and have zero negative performance impact.

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Verification & Testing Plan

1. Unit Tests: Ran the full test suite in a local virtual environment:

   PYTHONPATH=src .venv/bin/python manage.py test

   • Results: All 77 unit tests passed successfully with zero warnings/errors.
2. Startup Checks: Ran manual system check and migration tasks:

   PYTHONPATH=src .venv/bin/python manage.py check
   PYTHONPATH=src .venv/bin/python manage.py migrate

   • Results: Clean startup with zero warning output.

[peterfarrell]

@ricosalomar looks great -- generated some unit tests to cover some of the code (we never had any for apps.py)