test(backend): add ArchUnit domain boundary enforcement (Rules 1–4)

Rules enforced:
- Rule 1: no @RestController may inject a JpaRepository directly (preserves @RequirePermission AOP enforcement)
- Rule 2: @Service classes access only their own domain's repositories, never a foreign domain's
- Rule 3: no @Configuration class (except @SpringBootApplication) in domain packages
- Rule 4: all @Entity classes reside in a domain package

Rule 5 (URL prefix per controller domain) deferred — tracked in #427.

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

backend/pom.xml

@@ -108,6 +108,12 @@
 			<groupId>org.awaitility</groupId>
 			<artifactId>awaitility</artifactId>
 			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>com.tngtech.archunit</groupId>
+			<artifactId>archunit-junit5</artifactId>
+			<version>1.3.0</version>
+			<scope>test</scope>
 		</dependency>
 				<!-- Excel Bearbeitung (Apache POI) -->
 		<dependency>