feat(deps): support Django 6.0 and 6.1 - #3

What changed

• django >=4.2,<6.0 → >=4.2,<6.2; django-stubs ^5.2.0 → ^6.0.5; ruff ^0.15.22 → ^0.17.3.
• Added the Django 6.0 and 6.1 classifiers.
• tox pairs each Python with the Django releases that support it, following vintasend-django's tox.ini. The lock still resolves Django 5.2 because Django 6 needs Python 3.12, so the factor pins are what actually test 6.x.
• The CI test job runs poetry run tox instead of pytest, so PRs exercise the Django matrix rather than only the locked version (publish.yml already ran tox).
• Moved None to the end of two type-union aliases for ruff 0.17's RUF036; no behaviour change.
• Regenerated poetry.lock with Poetry 2.0.0 so every dependency resolves to the newest release its range allows on the supported Python floor.
• Ran py310-django42, py312-django60 and py313-django61: all 181 tests pass.

Breaking: no. The Django range widens. The Python matrix stays at 3.10–3.13; adding 3.14 is left for a separate change.

Verification

ruff check, ruff format --check, mypy and pytest pass locally against the new lock.

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