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/app.module.ts

@@ -8,6 +8,7 @@ import { ForcePasswordChangeInterceptor } from './auth/interceptors/force-passwo
 import { HealthModule } from './health/health.module';
 import { LdapModule } from './ldap/ldap.module';
 import { MailModule } from './mail/mail.module';
+import { DomaincheckModule } from './domaincheck/domaincheck.module';
 import { ModuleRegistryModule } from './module-registry/module-registry.module';
 import { PrismaModule } from './prisma/prisma.module';
 import { TenantMiddleware } from './tenant/tenant.middleware';
@@ -25,6 +26,7 @@ import { UserModule } from './user/user.module';
     MailModule,
     LdapModule,
     ModuleRegistryModule,
+    DomaincheckModule,
   ],
   providers: [
     // Global JWT guard: all routes require auth unless @Public()