fix(billing): create Stripe customer before redirecting to billing portal (#206)

## Summary

- Users without a Stripe customer ID (`stripe_id = null`) would hit an
`InvalidCustomer` exception when visiting the billing portal
- Added a `hasStripeId()` check before calling
`redirectToBillingPortal()`, creating the Stripe customer on-the-fly if
needed
- Added two tests covering both branches (with and without an existing
Stripe customer ID)

app/Http/Controllers/SubscriptionController.php

@@ -136,6 +136,12 @@ class SubscriptionController extends Controller
                 ->withErrors(['demo' => 'Billing management is not available on the demo account.']);
         }
 
-        return $request->user()->redirectToBillingPortal(route('settings.billing'));
+        $user = $request->user();
+
+        if (! $user->hasStripeId()) {
+            $user->createAsStripeCustomer();
+        }
+
+        return $user->redirectToBillingPortal(route('settings.billing'));
     }
 }

tests/Feature/SubscriptionTest.php

@@ -1,11 +1,13 @@
 <?php
 
+use App\Http\Middleware\HandleInertiaRequests;
 use App\Models\Account;
 use App\Models\AccountBalance;
 use App\Models\BankingConnection;
 use App\Models\Category;
 use App\Models\Transaction;
 use App\Models\User;
+use Illuminate\Http\RedirectResponse;
 use Laravel\Pennant\Feature;
 
 beforeEach(function () {
@@ -277,3 +279,35 @@ test('paywall shows canUseFreePlan false when open banking is not active', funct
             ->where('canUseFreePlan', false)
         );
 });
+
+test('billing portal creates stripe customer when user has no stripe id', function () {
+    $user = Mockery::mock(User::class)->shouldIgnoreMissing();
+    $user->shouldReceive('isDemoAccount')->andReturn(false);
+    $user->shouldReceive('hasStripeId')->once()->andReturn(false);
+    $user->shouldReceive('createAsStripeCustomer')->once();
+    $user->shouldReceive('redirectToBillingPortal')
+        ->with(route('settings.billing'))
+        ->once()
+        ->andReturn(new RedirectResponse(route('settings.billing')));
+
+    $this->withoutMiddleware(HandleInertiaRequests::class);
+    $this->actingAs($user);
+
+    $this->get(route('settings.billing.portal'))->assertRedirect();
+});
+
+test('billing portal skips stripe customer creation when user already has a stripe id', function () {
+    $user = Mockery::mock(User::class)->shouldIgnoreMissing();
+    $user->shouldReceive('isDemoAccount')->andReturn(false);
+    $user->shouldReceive('hasStripeId')->once()->andReturn(true);
+    $user->shouldNotReceive('createAsStripeCustomer');
+    $user->shouldReceive('redirectToBillingPortal')
+        ->with(route('settings.billing'))
+        ->once()
+        ->andReturn(new RedirectResponse(route('settings.billing')));
+
+    $this->withoutMiddleware(HandleInertiaRequests::class);
+    $this->actingAs($user);
+
+    $this->get(route('settings.billing.portal'))->assertRedirect();
+});