fix loadtest p2p preflight replay

loadtest/im-pressure/httpapi.go

@@ -216,8 +216,11 @@ type p2pResponse struct {
 
 func (a *apiClient) ensureP2P(ctx context.Context, source, target AccountRecord) (*p2pResponse, error) {
 	var result p2pResponse
+	// EnsureP2P is itself idempotent and must execute on every preflight so it
+	// can repair subscriptions and refresh a loaded topic. A stable HTTP
+	// idempotency key would replay an old response and skip that repair.
 	err := a.do(ctx, http.MethodPost, "/v1/im/conversations/p2p", source.Token,
-		fmt.Sprintf("loadtest-p2p-%s-%04d", source.Machine, source.Index),
+		"",
 		map[string]any{"peer_user_id": target.UserID}, &result)
 	return &result, err
 }