Chore: Move unittest directory setup onto the shared fixture (#14172)

The unittest side of the suite built its temp directory tree with
tempfile.mkdtemp and a manually enabled override_settings, cleaned up only if
tearDown ran. That is now gone. DirectoriesMixin lives alongside the layout it
bridges and does nothing but hand the paperless_dirs fixture to TestCase
subclasses as self.dirs, so both halves of the suite get the same twelve
settings, the same directory shapes and cleanup owned by tmp_path.

The mixin moves to paperless_testing.dirs rather than staying in the documents
test utilities, because modules in paperless and paperless_mail import it
across the app boundary. The thirty-eight consuming modules change only their
import line; self.dirs.scratch_dir and its siblings keep working.

src/paperless_testing/dirs.py

@@ -14,6 +14,7 @@ from tempfile import TemporaryDirectory
 from typing import TYPE_CHECKING
 from typing import TypedDict
 
+import pytest
 from django.test import override_settings
 
 if TYPE_CHECKING:
@@ -129,3 +130,17 @@ def paperless_environment() -> Iterator[PaperlessDirs]:
                 yield dirs
             finally:
                 reset_backend()
+
+
+class DirectoriesMixin:
+    """Bridges the directory fixture to unittest TestCase subclasses and to
+    plain pytest test classes that want `self.dirs`.
+
+    Delete once no class depends on it.
+    """
+
+    dirs: PaperlessDirs
+
+    @pytest.fixture(autouse=True)
+    def _paperless_dirs(self, paperless_dirs: PaperlessDirs) -> None:
+        self.dirs = paperless_dirs