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>
