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.module.ts

@@ -0,0 +1,38 @@
+import { Logger, Module, OnModuleInit } from '@nestjs/common';
+import { ModuleRegistryModule } from '../module-registry/module-registry.module';
+import { ModuleRegistryService } from '../module-registry/module-registry.service';
+import { DomaincheckController } from './domaincheck.controller';
+import { seedDomaincheckModule } from './domaincheck.seed';
+import { DomaincheckService } from './domaincheck.service';
+
+/**
+ * NestJS module for the Domaincheck feature.
+ *
+ * Provides DNS-based domain availability checking across TLD variants.
+ * Seeds itself into the module registry on application startup via
+ * OnModuleInit lifecycle hook.
+ *
+ * Imports ModuleRegistryModule for guard access (UseModule decorator)
+ * and seed function. PrismaModule is global, no explicit import needed.
+ */
+@Module({
+  imports: [ModuleRegistryModule],
+  controllers: [DomaincheckController],
+  providers: [DomaincheckService],
+})
+export class DomaincheckModule implements OnModuleInit {
+  private readonly logger = new Logger(DomaincheckModule.name);
+
+  constructor(
+    private readonly moduleRegistryService: ModuleRegistryService,
+  ) {}
+
+  async onModuleInit(): Promise<void> {
+    try {
+      await seedDomaincheckModule(this.moduleRegistryService);
+      this.logger.log('Domaincheck module seeded in registry');
+    } catch (error) {
+      this.logger.error('Failed to seed domaincheck module', error);
+    }
+  }
+}