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>

migrations/Version20260319193353.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260319193353 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return '';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE "user" ADD stripe_account_id VARCHAR(255) DEFAULT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('ALTER TABLE "user" DROP stripe_account_id');
+    }
+}