test(auth): add test_existing_endpoints_require_auth + per-test AUTH_DISABLED flips

- New test_existing_endpoints_require_auth.py: spot-check that existing
  /api/* endpoints now require auth (gated via Depends(matrix_gate)) when
  AUTH_DISABLED is False. Health remains public.

- conftest.py: flip AUTH_DISABLED=True for the suite so the legacy
  pre-auth tests keep passing without login. Auth tests flip it back
  off via their own autouse fixture (now patched to use monkeypatch
  for cleanup).

Verified: 53 auth tests pass; 222 pre-existing non-auth failures are
unchanged.

backend/tests/test_auth_admin.py

@@ -13,7 +13,11 @@ from cyclone.db import SessionLocal, User
 
 
 @pytest.fixture(autouse=True)
-def _clear():
+def _clear(monkeypatch):
+    # conftest sets AUTH_DISABLED=True so pre-auth tests keep passing
+    # without a login. The auth-admin tests need the real auth path
+    # exercised, so flip it back off here.
+    monkeypatch.setattr("cyclone.auth.deps.AUTH_DISABLED", False)
     with SessionLocal()() as db:
         db.execute(delete(DbSession))
         db.execute(delete(User))