refactor(middleware): use Account::isOnTrial() instead of onGenericTrial()

Centralizes the 'is the user on any kind of trial?' question in the model.
Equivalent behavior — Cashier's subscribed() already includes trialing
subscriptions — but semantically cleaner and easier to extend.

app/Http/Middleware/App/EnsureAccountReady.php

@@ -27,7 +27,7 @@ public function handle(Request $request, Closure $next): Response
         if (! config('trypost.self_hosted')) {
             $hasAccess = $account && (
                 $account->subscribed(Account::SUBSCRIPTION_NAME)
-                || $account->onGenericTrial()
+                || $account->isOnTrial()
             );
 
             if (! $hasAccess) {