refactor: allow yearly to monthly swaps in BillingController

Drops the abort_if guard that blocked switching from a yearly billing
cadence to monthly. The product decision was reversed — users should
be free to move in either direction without going through support.

Removes the corresponding 'swap blocks yearly to monthly downgrade'
test.

app/Http/Controllers/App/BillingController.php

@@ -143,16 +143,6 @@ public function swap(Request $request, Plan $plan): RedirectResponse
             'Invalid price for this plan',
         );
 
-        $currentPlan = $account->plan;
-        $isOnYearly = $currentPlan && $subscription->stripe_price === $currentPlan->stripe_yearly_price_id;
-        $isTargetMonthly = $priceId === $plan->stripe_monthly_price_id;
-
-        abort_if(
-            $isOnYearly && $isTargetMonthly,
-            422,
-            'Cannot downgrade from yearly to monthly billing.',
-        );
-
         $authorization = Gate::inspect('swapPlan', [$account, $plan]);
 
         if ($authorization->denied()) {

tests/Feature/BillingControllerTest.php

@@ -244,33 +244,6 @@
     ]));
 });
 
-test('swap blocks yearly to monthly downgrade', function () {
-    config(['trypost.self_hosted' => false]);
-
-    $plan = Plan::where('slug', 'max')->first();
-    $plan->update([
-        'stripe_monthly_price_id' => 'price_monthly',
-        'stripe_yearly_price_id' => 'price_yearly',
-    ]);
-    $this->account->update(['plan_id' => $plan->id]);
-
-    $this->user->unsetRelation('account');
-
-    $this->account->subscriptions()->create([
-        'type' => Account::SUBSCRIPTION_NAME,
-        'stripe_id' => 'sub_test_'.fake()->uuid(),
-        'stripe_status' => 'active',
-        'stripe_price' => 'price_yearly',
-    ]);
-
-    $response = $this->actingAs($this->user)
-        ->post(route('app.billing.swap', $plan), [
-            'price_id' => 'price_monthly',
-        ]);
-
-    $response->assertStatus(422);
-});
-
 test('swap rejects invalid price_id for plan', function () {
     config(['trypost.self_hosted' => false]);