fix: make Permission enum complete and grant all permissions to admin group

Three related issues:
- READ_ALL was stored in the DB but missing from the Permission enum
- ADMIN_USER, ADMIN_TAG and ADMIN_PERMISSION were in the enum and used
  in controllers but never granted to any user, making those endpoints
  permanently inaccessible
- No runtime signal when a DB permission string drifts from the enum

Changes:
- Add READ_ALL to Permission enum
- Grant all six permissions to the Administrators group in DataInitializer
- Warn in CustomUserDetailsService when a DB permission string has no
  matching Permission enum value

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

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

@@ -49,7 +49,7 @@ public class DataInitializer {
                 // 1. Admin Gruppe erstellen
                 UserGroup adminGroup = UserGroup.builder()
                         .name("Administrators")
-                        .permissions(Set.of("ADMIN", "READ_ALL", "WRITE_ALL"))
+                        .permissions(Set.of("ADMIN", "READ_ALL", "WRITE_ALL", "ADMIN_USER", "ADMIN_TAG", "ADMIN_PERMISSION"))
                         .build();
                 groupRepository.save(adminGroup);