Add Groups management and split Admin navigation

- New backend: Group + GroupMembership models, schemas, CRUD router at
  /api/admin/groups (list, create, get detail, update, delete, add/remove members)
- New Alembic migration: groups and group_memberships tables
- Frontend: Admin sidebar item is now an expandable accordion with
  Users and Groups sub-items; AdminPage redirects to /admin/users;
  new AdminUsersPage and AdminGroupsPage with inline member management panel
- API client: 7 new group functions + TypeScript types

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

backend/app/models/__init__.py

@@ -1,4 +1,5 @@
+from app.models.group import Group, GroupMembership
 from app.models.profile import Profile
 from app.models.user import User
 
-__all__ = ["User", "Profile"]
+__all__ = ["User", "Profile", "Group", "GroupMembership"]