fix(user): findOrCreate Administrators group instead of blind-INSERT (#518)

Closes #518.

UserDataInitializer.initAdminUser was doing groupRepository.save(adminGroup)
unconditionally. If a previous boot had seeded the group but failed
before creating the admin user (or if the operator deleted just the
admin row to retry with a corrected APP_ADMIN_USERNAME), the next
seed attempt violated user_groups_name_key and aborted the context.

Switch to the same findByName(...).orElseGet(...) pattern initE2EData
already uses for the "Leser" group.

Tests in AdminSeedFailClosedTest:
- reuses_existing_Administrators_group_when_seeding_a_new_admin
- creates_Administrators_group_when_seeding_admin_on_a_fresh_database
Plus updated existing tests to stub groupRepository.save now that the
seed path also exercises it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/user/UserDataInitializer.java

@@ -67,11 +67,16 @@ public class UserDataInitializer {
                 }
                 log.info("Kein Admin-User '{}' gefunden. Erstelle Default-Admin...", adminEmail);
 
-                UserGroup adminGroup = UserGroup.builder()
-                        .name("Administrators")
-                        .permissions(Set.of("ADMIN", "READ_ALL", "WRITE_ALL", "ANNOTATE_ALL", "ADMIN_USER", "ADMIN_TAG", "ADMIN_PERMISSION"))
-                        .build();
-                groupRepository.save(adminGroup);
+                // Reuse the Administrators group if it already exists (e.g. a
+                // previous boot seeded the group but failed before creating
+                // the admin user, or the operator deleted just the user row
+                // to retry the seed with a new email). Blind-INSERTing would
+                // violate user_groups_name_key and abort the context. See #518.
+                UserGroup adminGroup = groupRepository.findByName("Administrators")
+                        .orElseGet(() -> groupRepository.save(UserGroup.builder()
+                                .name("Administrators")
+                                .permissions(Set.of("ADMIN", "READ_ALL", "WRITE_ALL", "ANNOTATE_ALL", "ADMIN_USER", "ADMIN_TAG", "ADMIN_PERMISSION"))
+                                .build()));
 
                 AppUser admin = AppUser.builder()
                         .email(adminEmail)