Add SCUM capability negotiation gate

platform/api/scum_handlers.go

@@ -6,6 +6,19 @@ import (
 	"browser.local/platform/dto"
 )
 
+func (h *coreHandlers) serverSCUMCapabilities(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		writeMethodNotAllowed(w, http.MethodGet)
+		return
+	}
+	negotiation, err := h.core.NegotiateSCUMCapabilitiesForSession(bearerToken(r), r.PathValue("id"))
+	if err != nil {
+		writeServiceError(w, err)
+		return
+	}
+	writeJSON(w, http.StatusOK, dto.SCUMCapabilityNegotiationFromDomain(negotiation))
+}
+
 func (h *coreHandlers) serverSCUMSchemaProbe(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodPost {
 		writeMethodNotAllowed(w, http.MethodPost)