Add function-scoped auth_client fixture for auth tests

- Add auth_client fixture (function-scoped) for authentication tests
- Update all tests in test_auth_api.py to use auth_client
- Prevents auth tests from polluting the shared integration_client session
- Each auth test gets a fresh client, avoiding state leakage

backend/tests/conftest.py

@@ -249,6 +249,26 @@ def integration_client():
         yield client
 
 
+@pytest.fixture
+def auth_client():
+    """
+    Create a function-scoped test client for authentication tests.
+
+    Unlike integration_client (session-scoped), this creates a fresh client
+    for each test. Use this for tests that manipulate authentication state
+    (login, logout, cookie clearing) to avoid polluting other tests.
+
+    Environment variables:
+        ORCHARD_TEST_URL: Base URL of the Orchard server (default: http://localhost:8080)
+    """
+    import httpx
+
+    base_url = os.environ.get("ORCHARD_TEST_URL", "http://localhost:8080")
+
+    with httpx.Client(base_url=base_url, timeout=30.0) as client:
+        yield client
+
+
 @pytest.fixture
 def unique_test_id():
     """Generate a unique ID for test isolation."""