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/dto/check-domain.dto.ts

@@ -0,0 +1,34 @@
+import {
+  ArrayMaxSize,
+  IsArray,
+  IsNotEmpty,
+  IsOptional,
+  IsString,
+  Matches,
+} from 'class-validator';
+
+/**
+ * DTO for domain availability check requests.
+ *
+ * Per T-03-05: domain input is validated with regex — only alphanumeric
+ * characters and hyphens allowed, no dots, max 63 chars (DNS label limit).
+ */
+export class CheckDomainDto {
+  @IsString()
+  @IsNotEmpty()
+  @Matches(/^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$/, {
+    message:
+      'Domain must be a valid label: alphanumeric and hyphens only, 1-63 characters, cannot start or end with a hyphen',
+  })
+  domain!: string;
+
+  /**
+   * Optional list of TLDs to check. If omitted, defaults are used.
+   * Per T-03-06: max 10 TLDs per request to prevent DoS.
+   */
+  @IsOptional()
+  @IsArray()
+  @IsString({ each: true })
+  @ArrayMaxSize(10, { message: 'Maximum 10 TLDs per request' })
+  tlds?: string[];
+}