feat: seed eBay platform row via migration

INSERT INTO app.platforms WHERE NOT EXISTS — idempotent, safe to run
on existing installations. Fixes "eBay-Platform nicht konfiguriert" error
when opening the eBay category config CRUD.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/Version20260520110000.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260520110000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Seed eBay platform row if not present';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql(<<<'SQL'
+            INSERT INTO app.platforms (id, type, label, config)
+            SELECT gen_random_uuid(), 'ebay', 'eBay', '[]'
+            WHERE NOT EXISTS (SELECT 1 FROM app.platforms WHERE type = 'ebay')
+        SQL);
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql("DELETE FROM app.platforms WHERE type = 'ebay'");
+    }
+}