test(django): Remove @pytest.mark.forked dependency from Django integration tests

## Background

The Django integration test suite uses `@pytest.mark.forked` (via `pytest-forked`) on 52 tests across 5 files to isolate global state. This dates back to #522 ("Less boxed testing") and was a pragmatic substitute for proper teardown.

## Problem

`pytest-forked` is actively harmful in some scenarios:

- **Python 3.12 + asyncio + GitHub Actions:** Forking a multi-threaded process and then running an asyncio loop in the child causes silent hangs (the loop inherits dead threads). This consumed the full 30-minute CI budget on the Django ASGI tests until #6410 dropped `forked` from those 10 tests.
- **Slow:** Fork-per-test multiplies suite runtime.
- **Hides real isolation bugs:** Tests pass under fork but would fail under proper sequential execution, masking integration setup that isn't idempotent.

## Goal

Remove `@pytest.mark.forked` from every test under `tests/integrations/django/` and replace it with explicit isolation.

## Proposed approach

1. Extend (or autouse-wrap) the `sentry_init` fixture so the non-forked path also resets:
- `sentry_sdk.integrations._processed_integrations`
- `sentry_sdk.integrations._installed_integrations`
2. Add a `reset_django_state` fixture that snapshots and restores:
- `settings.MIDDLEWARE`
- Django signal receivers (`request_started`, `request_finished`, etc.)
- Anything else `DjangoIntegration.setup_once` connects
3. Audit `DjangoIntegration.setup_once` (and submodule patches under `sentry_sdk/integrations/django/`) for non-idempotent module-level patching.
4. Migrate test files one at a time: drop the marker, run, fix the resulting leaks, repeat.

## Current state

Files still using `@pytest.mark.forked` under `tests/integrations/django/` (count via `grep -rc`):

- `test_basic.py` (13)
- `test_cache_module.py` (14)
- `test_db_query_data.py` (10)
- `test_db_transactions.py` (12)
- `test_data_scrubbing.py` (3)

The ASGI subset (`tests/integrations/django/asgi/test_asgi.py`) was removed in #6410.