C3: configurable maxHistory (default 50)

nix/hm-module.nix

@@ -108,6 +108,11 @@ in
             default = 10;
             description = "Maximum visible notifications in the notification center before scrolling.";
           };
+          maxHistory = lib.mkOption {
+            type = lib.types.int;
+            default = 50;
+            description = "Maximum notifications kept in history.";
+          };
         };
         bluetooth = moduleOpt "bluetooth" (intervalOpt 5000);
         network = moduleOpt "network" (intervalOpt 5000);