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/resources/db/migration/V2__admin_group_seed.sql

@@ -0,0 +1,7 @@
+-- Built-in Admins group
+INSERT INTO groups (id, name) VALUES
+    ('00000000-0000-0000-0000-000000000010', 'Admins');
+
+-- Assign ADMIN role to Admins group
+INSERT INTO group_roles (group_id, role_id) VALUES
+    ('00000000-0000-0000-0000-000000000010', '00000000-0000-0000-0000-000000000004');