Clear Web UI session on 401 and keep API/Ingress errors as JSON.

Include the missing resource id in not-found messages, expire auth to Guest
after 401, and document Ingress annotations so nginx does not rewrite 404/405
bodies into branded HTML.

tests/unit/test_extended_handlers.py

@@ -176,5 +176,5 @@ async def test_missing_node_returns_404() -> None:
     pool.node_exists = False
     handler = registry.get("/nodes/{node}/storage", "GET")
     assert handler is not None
-    with pytest.raises(ApiError, match="node does not exist"):
+    with pytest.raises(ApiError, match=r"No such node \('missing'\)"):
         await handler(_request(pool), {"values": {"node": "missing"}})