refactor(search): replace context manager smell with explicit open/close lifecycle

TantivyBackend now uses open()/close()/_ensure_open() instead of __enter__/__exit__.
get_backend() tracks _backend_path and auto-reinitializes when settings.INDEX_DIR
changes, fixing the xdist/override_settings isolation bug where parallel workers
would share a stale singleton pointing at a deleted index directory.

Test fixtures use in-memory indices (path=None) for speed and isolation.
Singleton behavior covered by TestSingleton in test_backend.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/tests/search/conftest.py

@@ -23,8 +23,11 @@ def index_dir(tmp_path: Path, settings: SettingsWrapper) -> Path:
 
 
 @pytest.fixture
-def backend(index_dir: Path) -> Generator[TantivyBackend, None, None]:
-    b = TantivyBackend()
-    with b:
+def backend() -> Generator[TantivyBackend, None, None]:
+    b = TantivyBackend()  # path=None → in-memory index
+    b.open()
+    try:
         yield b
-    reset_backend()
+    finally:
+        b.close()
+        reset_backend()

src/documents/tests/search/test_backend.py

@@ -6,6 +6,8 @@ from documents.models import CustomFieldInstance
 from documents.models import Document
 from documents.models import Note
 from documents.search._backend import TantivyBackend
+from documents.search._backend import get_backend
+from documents.search._backend import reset_backend
 
 pytestmark = [pytest.mark.search, pytest.mark.django_db]
 
@@ -161,6 +163,37 @@ class TestMoreLikeThis:
         assert 50 not in returned_ids  # Original document excluded
 
 
+class TestSingleton:
+    """Test get_backend() and reset_backend() singleton lifecycle."""
+
+    @pytest.fixture(autouse=True)
+    def _clean(self):
+        reset_backend()
+        yield
+        reset_backend()
+
+    def test_returns_same_instance_on_repeated_calls(self, index_dir):
+        assert get_backend() is get_backend()
+
+    def test_reinitializes_when_index_dir_changes(self, tmp_path, settings):
+        settings.INDEX_DIR = tmp_path / "a"
+        (tmp_path / "a").mkdir()
+        b1 = get_backend()
+
+        settings.INDEX_DIR = tmp_path / "b"
+        (tmp_path / "b").mkdir()
+        b2 = get_backend()
+
+        assert b1 is not b2
+        assert b2._path == tmp_path / "b"
+
+    def test_reset_forces_new_instance(self, index_dir):
+        b1 = get_backend()
+        reset_backend()
+        b2 = get_backend()
+        assert b1 is not b2
+
+
 class TestFieldHandling:
     """Test handling of various document fields."""