test(archunit): fix foreignJpaRepositoryFor exact-segment matching

Replace substring contains() with a regex exact-segment match so a
domain whose name is a substring of another (e.g. "tag" in "tagging")
cannot silently escape the predicate and produce a false negative.

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

backend/src/test/java/org/raddatz/familienarchiv/shared/ArchitectureTest.java

@@ -118,11 +118,15 @@ class ArchitectureTest {
     // Tracked in: http://heim-nas:3005/marcel/familienarchiv/issues/427
 
     private static DescribedPredicate<JavaClass> foreignJpaRepositoryFor(String ownDomain) {
+        // Exact-segment match: prevents a domain name that is a substring of another
+        // (e.g. "tag" inside "tagging") from silently escaping the predicate.
+        // The pattern matches the domain as a complete path segment, with an optional sub-package.
+        String ownPackagePattern = ".*\\.familienarchiv\\." + ownDomain + "(\\..+)?$";
         return new DescribedPredicate<JavaClass>("be a JPA repository from a domain other than " + ownDomain) {
             @Override
             public boolean test(JavaClass clazz) {
                 return clazz.isAssignableTo(JpaRepository.class)
-                       && !clazz.getPackageName().contains("." + ownDomain);
+                       && !clazz.getPackageName().matches(ownPackagePattern);
             }
         };
     }