fix: use https base_url in test client so Secure cookies propagate automatically

httpx's cookie jar only auto-attaches Secure cookies to https:// requests.
Switching the ASGITransport client fixture's base_url from http://test to
https://test (no real socket is opened either way) makes it behave like a
browser talking to the Cloudflare-Tunnel-terminated HTTPS edge in
production, eliminating the need for manual client.cookies.set(...)
re-injection workarounds in test_auth.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/tests/conftest.py

@@ -35,5 +35,5 @@ app.dependency_overrides[get_db] = _override_get_db
 @pytest_asyncio.fixture
 async def client():
     transport = ASGITransport(app=app)
-    async with AsyncClient(transport=transport, base_url="http://test") as ac:
+    async with AsyncClient(transport=transport, base_url="https://test") as ac:
         yield ac