fix(e2e): use username check instead of count() for admin user creation

When the e2e profile is active, initE2EData (which creates a reader user)
can run before initAdminUser. The old count() == 0 guard then skips admin
creation entirely, causing every login test to fail with 401.

Switch to findByUsername(adminUsername).isEmpty() so the admin is created
regardless of which CommandLineRunner runs first.

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

backend/src/main/java/org/raddatz/familienarchiv/config/DataInitializer.java

@@ -43,8 +43,8 @@ public class DataInitializer {
     @Bean
     public CommandLineRunner initAdminUser(PasswordEncoder passwordEncoder) {
         return args -> {
-            if (userRepository.count() == 0) {
-                log.info("Keine User gefunden. Erstelle Default-Admin...");
+            if (userRepository.findByUsername(adminUsername).isEmpty()) {
+                log.info("Kein Admin-User '{}' gefunden. Erstelle Default-Admin...", adminUsername);
 
                 // 1. Admin Gruppe erstellen
                 UserGroup adminGroup = UserGroup.builder()