fix(deps): pin pytest-django to 4.11.1 to unblock CI

[WilfordGrimley]

Description

pytest-django~=4.11 in MPCAutofill/requirements.txt resolved to 4.12.0,
which raises AttributeError: type object 'PytestDjangoTestCase' has no attribute '_pre_setup_ran_eagerly' inside its own fixture machinery. This
fires at setup of every test in
cardpicker/tests/test_artbox_exemplar_backfill.py, failing Backend tests
on every open PR (confirmed on #703 and #717, identical signature) and on
master's next run. No project code causes or can fix this — the attribute
belongs to pytest-django's own internals, which changed between 4.11.x and
4.12.0.

This PR pins to pytest-django==4.11.1 (the newest 4.11.x release, verified
working) with an inline comment explaining what the pin protects against, so
the resolution is deliberate rather than whatever upstream published most
recently.

Other unpinned/loosely-pinned specifiers with the same hazard

Not touched here, scoped out to keep this diff reviewable, listed for a
follow-up:

• django-allauth[socialaccount]~=65.4 — two-component ~=, admits any 65.x
• factory-boy~=3.2 — two-component ~=, admits any 3.x
• freezegun~=1.2 — two-component ~=, admits any 1.x
• pytest~=9.0 — two-component ~=, admits any 9.x
• syrupy~=5.1 — two-component ~=, admits any 5.x
• pre-commit — fully unpinned
• testcontainers[postgres,elasticsearch] — fully unpinned
• tzdata — fully unpinned

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate. (N/A — dependency pin only, no test/behavior change)
• I have manually tested my changes as follows:
  • Built a throwaway virtualenv under /tmp, installed MPCAutofill/requirements.txt with the new pin, confirmed pytest-django-4.11.1 resolved (not 4.12.0).
  • Ran cardpicker/tests/test_artbox_exemplar_backfill.py (the file that errors at setup under 4.12.0) in that venv: 33 passed.
  • Ran the full cardpicker/ suite in the same venv: 3655 passed, 11 skipped, 0 failed — no regressions from the downgrade.
• I have updated any relevant documentation or created new documentation where appropriate. (N/A — no user- or admin-facing behavior change)

Task-end checks

• Wiki: N/A — no user- or admin-facing change.
• Extractable-primitives ledger: N/A — no primitive extracted/removed.
• Constant-rename equivalence: N/A — no constant renamed/moved.