feat(03-02): add domaincheck backend - DNS service, API endpoint, module seed

- CheckDomainDto with regex validation (T-03-05) and max 10 TLDs (T-03-06)
- DomaincheckService using node:dns/promises with 5s timeout per lookup
- POST /modules/domaincheck/check protected by UseModule guard (T-03-08)
- DomaincheckModule seeds itself into registry on startup via OnModuleInit
- Default TLDs: de, com, net, org (D-03)

apps/api/src/domaincheck/domaincheck.seed.ts

@@ -0,0 +1,25 @@
+import { ModuleRegistryService } from '../module-registry/module-registry.service';
+
+/**
+ * Seeds the domaincheck module into the module registry.
+ *
+ * Called during DomaincheckModule initialization to ensure the
+ * "domaincheck" module record exists in the database.
+ *
+ * Per D-09: category is 'domain-tools'.
+ */
+export async function seedDomaincheckModule(
+  moduleRegistryService: ModuleRegistryService,
+): Promise<void> {
+  await moduleRegistryService.seedModule({
+    slug: 'domaincheck',
+    name: 'Domaincheck',
+    version: '1.0.0',
+    category: 'domain-tools',
+    description: {
+      de: 'Domain-Verfuegbarkeit pruefen',
+      en: 'Check domain availability',
+    },
+    isSystem: true,
+  });
+}