feat(rest): 501 stub for /v1/actions/snapshot/multi

golinstor uses this for atomic multi-RD group snapshots. Cozystack
runs PVCs as independent RDs with independent snapshot lifecycles,
so the multi-RD consistency point isn't needed. Returning 501 keeps
the contract honest and lets the client fall back to per-RD
snapshots without ambiguity.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps

pkg/rest/server.go

@@ -121,6 +121,7 @@ func (s *Server) buildMux() *http.ServeMux {
 	s.registerSOSReport(mux)
 	s.registerRemotes(mux)
 	s.registerNodeConnections(mux)
+	s.registerSnapshotMulti(mux)
 
 	return mux
 }

pkg/rest/snapshot_multi.go

@@ -0,0 +1,37 @@
+/*
+Copyright 2026 Cozystack contributors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package rest
+
+import (
+	"net/http"
+)
+
+// registerSnapshotMulti wires `POST /v1/actions/snapshot/multi`. golinstor
+// uses this to take an atomic group snapshot across several
+// ResourceDefinitions in a single call. Cozystack does not run multi-RD
+// applications that require a single consistency point on the LINSTOR
+// side (each PVC is its own RD with its own snapshot lifecycle), so this
+// is intentionally out-of-scope. Returning 501 keeps the contract clean
+// and lets clients fall back to per-RD snapshots.
+func (s *Server) registerSnapshotMulti(mux *http.ServeMux) {
+	mux.HandleFunc("POST /v1/actions/snapshot/multi", handleSnapshotMultiNotImplemented)
+}
+
+func handleSnapshotMultiNotImplemented(w http.ResponseWriter, _ *http.Request) {
+	writeError(w, http.StatusNotImplemented,
+		"multi-resource snapshot is not implemented; take per-RD snapshots instead")
+}