fix: degrade list_collections when folders are unavailable (403/404)

GET /folders/ is gated by the folder plan server-side, so on plans
without folder support list_collections raised CloudAPIError while
every sibling folder method (create_collection, get_or_create_collection,
delete_collection) degrades gracefully. Catch _FOLDER_UNAVAILABLE, emit
the one-time upgrade warning, and return []; transient errors still
propagate.

pageindex/backend/cloud.py

@@ -173,7 +173,13 @@ class CloudBackend:
         return None
 
     def list_collections(self) -> list[str]:
-        data = self._request("GET", "/folders/")
+        try:
+            data = self._request("GET", "/folders/")
+        except CloudAPIError as e:
+            if e.status_code in self._FOLDER_UNAVAILABLE:
+                self._warn_folder_upgrade()
+                return []
+            raise
         return [f["name"] for f in data.get("folders", []) or []]
 
     def delete_collection(self, name: str) -> None:

tests/test_cloud_backend.py

@@ -127,6 +127,28 @@ def test_folder_transient_error_propagates_and_is_not_cached(monkeypatch):
     assert "col" not in backend._folder_id_cache
 
 
+def test_list_collections_degrades_when_folders_unavailable(monkeypatch):
+    backend = CloudBackend(api_key="pi-test")
+
+    def fake_request(method, path, **kwargs):
+        raise CloudAPIError("Cloud API error 403: upgrade", status_code=403)
+
+    monkeypatch.setattr(backend, "_request", fake_request)
+    with pytest.warns(UserWarning, match="not available on this plan"):
+        assert backend.list_collections() == []
+
+
+def test_list_collections_propagates_transient_error(monkeypatch):
+    backend = CloudBackend(api_key="pi-test")
+
+    def fake_request(method, path, **kwargs):
+        raise CloudAPIError("Cloud API error 503: unavailable", status_code=503)
+
+    monkeypatch.setattr(backend, "_request", fake_request)
+    with pytest.raises(CloudAPIError):
+        backend.list_collections()
+
+
 # ── doc endpoints: 404 maps to DocumentNotFoundError (local parity) ──────────
 
 def test_doc_404_maps_to_document_not_found(monkeypatch):