Add ArchUnit rule: web adapter must not depend on outbound ports

Prevents future regressions where controllers bypass the application layer
and access repositories directly.

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

backend/src/test/java/de/fete/HexagonalArchitectureTest.java

@@ -60,4 +60,9 @@ class HexagonalArchitectureTest {
   static final ArchRule persistenceMustNotDependOnWeb = noClasses()
       .that().resideInAPackage("de.fete.adapter.out.persistence..")
       .should().dependOnClassesThat().resideInAPackage("de.fete.adapter.in.web..");
+
+  @ArchTest
+  static final ArchRule webAdapterMustNotDependOnOutboundPorts = noClasses()
+      .that().resideInAPackage("de.fete.adapter.in.web..")
+      .should().dependOnClassesThat().resideInAPackage("de.fete.domain.port.out..");
 }