Make delivery and integration failures explicit

Persist reminder presentations and retry state, atomically complete collapsed deliveries, fall back across away reaches, and block permanent failures visibly (V-715, V-678). Fail closed when enabled integrations lack credentials and keep remote arms explicitly dark (V-691). Give mavweb one sanitized, request-correlated error contract (V-689). Owner explicitly requested direct commits to master.

cmd/mavweb/models_test.go

@@ -149,6 +149,27 @@ type errBrokenModel struct{}
 
 func (errBrokenModel) Error() string { return "llm: server did not start" }
 
+type errPrivateModel struct{}
+
+func (errPrivateModel) Error() string { return "exec /private/llama-server: token rejected" }
+
+func TestModels_SwapFailureIsSanitizedAndTraceable(t *testing.T) {
+	core := &fakeModelCore{
+		swapErr: errPrivateModel{},
+		status:  ipc.ModelStatusResp{Model: "qwen3", ModelPath: "/m/old.gguf"},
+	}
+	w := modelsPOST(t, core, nil, false, "/m/cpt.gguf")
+	body := w.Body.String()
+	for _, want := range []string{"swap failed", string(problemModelsUnavailable), "request "} {
+		if !strings.Contains(body, want) {
+			t.Errorf("sanitized model error missing %q:\n%s", want, body)
+		}
+	}
+	if strings.Contains(body, "/private/llama-server") || strings.Contains(body, "token rejected") {
+		t.Errorf("model page disclosed the backend error:\n%s", body)
+	}
+}
+
 func TestModels_TotalFailureDoesNotSaySheIsStillAnswering(t *testing.T) {
 	// The load failed and so did the rollback: nothing is loaded. The page used
 	// to branch on RolledBack first and render "rolled back to  — she is still