feat(billing): apply Stripe tax rates to subscriptions (#325)

## Summary
Every subscription now gets Stripe tax rates attached automatically via
Cashier.

## Changes
- `config/subscriptions.php`: new `tax_rates` array, env
`STRIPE_TAX_RATES` (comma-separated), default
`txr_1TPfzrLRCmKA3oWMNWmkQeq2`
- `app/Models/User.php`: `taxRates()` reads from config — Cashier picks
it up automatically on `newSubscription()` checkout + subscription
creation
- `tests/Feature/SubscriptionTest.php`: 2 tests

## Applies to
- New checkout sessions (`SubscriptionController::checkout`)
- New subscriptions created via Cashier

## Existing subscriptions
Not updated automatically. To sync:
```php
$user->subscription('default')->syncTaxRates();
```

## Notes
Using hard-coded tax rate IDs (not Stripe Tax auto-calc). Switch to
`Cashier::calculateTaxes()` later if desired.

app/Models/User.php

@@ -165,6 +165,16 @@ class User extends Authenticatable implements HasLocalePreference, MustVerifyEma
         return $this->subscribed('default');
     }
 
+    /**
+     * The tax rates that should apply to the customer's subscriptions.
+     *
+     * @return array<int, string>
+     */
+    public function taxRates(): array
+    {
+        return config('subscriptions.tax_rates', []);
+    }
+
     public function isDemoAccount(): bool
     {
         return $this->email === config('app.demo.email');

config/subscriptions.php

@@ -131,6 +131,18 @@ return [
     |
     */
 
+    /*
+    |--------------------------------------------------------------------------
+    | Tax Rates
+    |--------------------------------------------------------------------------
+    |
+    | Stripe tax rate IDs applied to every subscription created via Cashier.
+    | Configure tax rates in your Stripe dashboard and reference them here.
+    |
+    */
+
+    'tax_rates' => array_values(array_filter(explode(',', (string) env('STRIPE_TAX_RATES', 'txr_1TPfzrLRCmKA3oWMNWmkQeq2')))),
+
     'promo' => [
         'enabled' => env('PROMO_ENABLED', true),
         'code' => 'FOUNDER',

tests/Feature/SubscriptionTest.php

@@ -272,6 +272,18 @@ test('paywall shows canUseFreePlan false when user has a bank connection', funct
         );
 });
 
+test('taxRates returns configured stripe tax rate ids', function () {
+    config(['subscriptions.tax_rates' => ['txr_test_1', 'txr_test_2']]);
+
+    $user = User::factory()->create();
+
+    expect($user->taxRates())->toBe(['txr_test_1', 'txr_test_2']);
+});
+
+test('taxRates config defaults to the production tax rate id', function () {
+    expect(config('subscriptions.tax_rates'))->toContain('txr_1TPfzrLRCmKA3oWMNWmkQeq2');
+});
+
 test('billing portal creates stripe customer when user has no stripe id', function () {
     $user = Mockery::mock(User::class)->shouldIgnoreMissing();
     $user->shouldReceive('isDemoAccount')->andReturn(false);