feat(auth): add server-backed demo sessions

The browser treated sessionStorage as the source of truth for the logged-in
user and never verified or invalidated the server-side session cookie: no
GET /api/v1/demo/session or POST /api/v1/demo/logout endpoint existed, and a
central 401 handler was defined but never wired up.

Add both endpoints; the session-check response is marked Cache-Control:
no-store to avoid the browser serving a stale "authenticated" response right
after logout. AuthProvider now verifies against the server on every mount
(sessionStorage only caches presentation state to avoid a login-screen
flash), subscribes to a central 401 listener on the API client, and
RequireAuth shows a loading state during verification instead of flashing
protected content or the wrong role.

backend/tests/test_auth.py

@@ -18,3 +18,32 @@ def test_operations_manager_can_reset_demo(ops_client):
     response = ops_client.post("/api/v1/demo/reset")
     assert response.status_code == 200
     assert response.json()["counts"]["vehicles"] == 50
+
+
+def test_session_endpoint_requires_authentication(client):
+    response = client.get("/api/v1/demo/session")
+    assert response.status_code == 401
+
+
+def test_session_endpoint_confirms_logged_in_user(ops_client):
+    response = ops_client.get("/api/v1/demo/session")
+    assert response.status_code == 200
+    body = response.json()
+    assert body["role"] == "operations_manager"
+    assert body["public_ref"] == "USR-OPS"
+
+
+def test_logout_invalidates_session(ops_client):
+    confirmed = ops_client.get("/api/v1/demo/session")
+    assert confirmed.status_code == 200
+
+    logout = ops_client.post("/api/v1/demo/logout")
+    assert logout.status_code == 200
+
+    after = ops_client.get("/api/v1/demo/session")
+    assert after.status_code == 401
+
+
+def test_logout_without_a_session_is_safe(client):
+    response = client.post("/api/v1/demo/logout")
+    assert response.status_code == 200