Add CI jobs for DjangoMessage tests with multiple Django versions

[lavr]

Problem

Django integration tests (emails/testsuite/django_/test_django_integrations.py) exist but never run in CI — django is not installed as a dependency, so the test module is skipped with ModuleNotFoundError.

Proposal

Add separate CI jobs that:

• Install Django as a test dependency
• Run only the Django integration tests (emails/testsuite/django_/)
• Test against 2–3 latest Django versions (4.2 LTS, 5.1, 5.2)
• Use a single Python version (3.12)

Why

DjangoMessage is part of the public API but has zero CI coverage. Regressions can go unnoticed across Django releases.