feat: seed built-in Admins group and assign admin users on login

- Add V2 Flyway migration to create built-in Admins group (id: ...0010) with ADMIN role
- Add ADMINS_GROUP_ID constant to SystemRole
- Add user to Admins group on successful local login alongside role assignment

cameleer3-server-app/src/main/java/com/cameleer3/server/app/security/UiAuthController.java

@@ -94,6 +94,7 @@ public class UiAuthController {
             userRepository.upsert(new UserInfo(
                     subject, "local", "", request.username(), Instant.now()));
             rbacService.assignRoleToUser(subject, SystemRole.ADMIN_ID);
+            rbacService.addUserToGroup(subject, SystemRole.ADMINS_GROUP_ID);
         } catch (Exception e) {
             log.warn("Failed to upsert local user to store (login continues): {}", e.getMessage());
         }