Chore: State the test directory layout in one place (#14171)

The temp directory layout used by the tests was written out four separate
times: once in the documents conftest, once in the paperless checks tests,
once in a fixture local to the NFC upload tests, and once in the helper
behind the old paperless_environment context manager. Each copy covered a
different subset of the settings, so which directories a test actually got
depended on which copy it happened to reach.

The layout now lives in paperless_testing.dirs. build_paperless_dirs owns
where things go and creates them, dirs_settings maps them onto Django
setting names and is pure, and a paperless_dirs fixture in the root conftest
applies that mapping through pytest-django's settings fixture so every app
can reach it. Tests that need a second environment part way through a test
body use the paperless_environment context manager from the same module,
which expresses the identical layout through override_settings. The three
redundant implementations and the old media settings fixture are gone, and
their consumers now take paperless_dirs.

src/conftest.py

@@ -5,8 +5,20 @@ this file is imported for every session,  so anything heavy belongs inside
 the fixture body that needs it.
 """
 
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
 import pytest
 
+if TYPE_CHECKING:
+    from collections.abc import Generator
+    from pathlib import Path
+
+    from pytest_django.fixtures import Settings
+
+    from paperless_testing.dirs import PaperlessDirs
+
 
 @pytest.fixture(scope="session", autouse=True)
 def faker_session_locale() -> str:
@@ -33,3 +45,27 @@ def _clear_content_type_caches() -> None:
 
     ContentType.objects.clear_cache()
     clear_ct_cache()
+
+
+@pytest.fixture
+def paperless_dirs(
+    tmp_path: Path,
+    settings: Settings,
+) -> Generator[PaperlessDirs, None, None]:
+    """The standard temp directory layout, applied to Django settings."""
+    from documents.search import reset_backend
+    from paperless_testing.dirs import build_paperless_dirs
+    from paperless_testing.dirs import dirs_settings
+
+    dirs = build_paperless_dirs(tmp_path)
+    for name, value in dirs_settings(dirs).items():
+        setattr(settings, name, value)
+
+    # Not directory settings, but they are needed alongside the layout by the
+    # sanity checker tests.
+    settings.IGNORABLE_FILES = {".DS_Store", "Thumbs.db", "desktop.ini"}
+    settings.APP_LOGO = ""
+
+    reset_backend()
+    yield dirs
+    reset_backend()