Add Stripe integration: webhook controller, service, sync command, Connect account

- Create StripeService: webhook sync, signature verification, save secret to .env.local
- Create StripeWebhookController with signature verification (400 on invalid)
- Create stripe:sync command to auto-create webhook endpoint via Stripe API
- Webhook listens to all events (configurable later)
- Save webhook secret automatically to .env.local on creation
- Add stripeAccountId field to User entity for Stripe Connect + migration
- Tests: StripeServiceTest (5), StripeWebhookControllerTest (2), StripeSyncCommandTest (1), UserTest updated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/Entity/UserTest.php

@@ -146,6 +146,17 @@ class UserTest extends TestCase
         self::assertSame(1.5, $user->getCommissionRate());
     }
 
+    public function testStripeAccountIdField(): void
+    {
+        $user = new User();
+
+        self::assertNull($user->getStripeAccountId());
+
+        $result = $user->setStripeAccountId('acct_1234567890');
+        self::assertSame($user, $result);
+        self::assertSame('acct_1234567890', $user->getStripeAccountId());
+    }
+
     public function testEmailVerificationFields(): void
     {
         $user = new User();