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>

changelog/2026-04-17_groups-and-admin-nav.md

@@ -0,0 +1,27 @@
+# 2026-04-17 — Groups management and Admin navigation split
+
+**Timestamp:** 2026-04-17T12:00:00Z
+
+## Summary
+
+Added a Groups system (backend models, API, migration) and split the Admin sidebar item into an expandable accordion with "Users" and "Groups" sub-navigation points.
+
+## Files Added / Modified / Deleted
+
+### Added
+- `backend/app/models/group.py` — `Group` and `GroupMembership` SQLAlchemy models
+- `backend/app/schemas/group.py` — Pydantic schemas: `GroupCreate`, `GroupUpdate`, `GroupOut`, `GroupDetailOut`, `GroupMemberOut`
+- `backend/app/routers/groups.py` — Admin CRUD endpoints for groups + member add/remove
+- `backend/alembic/versions/a3f9c2d14e87_add_groups_and_group_memberships.py` — Migration creating `groups` and `group_memberships` tables
+- `frontend/src/pages/AdminUsersPage.tsx` — User management page (extracted from AdminPage)
+- `frontend/src/pages/AdminGroupsPage.tsx` — Group management page with inline member panel
+
+### Modified
+- `backend/app/models/__init__.py` — Import `Group` and `GroupMembership`
+- `backend/app/main.py` — Mount `/api/admin/groups` router
+- `frontend/src/api/client.ts` — Added 7 group API functions and TypeScript types
+- `frontend/src/pages/AdminPage.tsx` — Now a simple redirect to `/admin/users`
+- `frontend/src/App.tsx` — Added routes `/admin/users` and `/admin/groups`
+- `frontend/src/components/Sidebar.tsx` — Admin item is now an expandable accordion with Users and Groups sub-items
+- `backend/STATUS.md` — Documented groups endpoints, models, updated future work
+- `frontend/STATUS.md` — Documented new routes, pages, API client functions