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/main.py

@@ -5,7 +5,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.core.config import settings
-from app.routers import admin, auth, categories_proxy, documents_proxy, profile, services, users
+from app.routers import admin, auth, categories_proxy, documents_proxy, groups, profile, services, users
 from app.routers import settings as settings_router
 from app.services.service_health import check_all, health_check_loop, register_services
 
@@ -41,6 +41,7 @@ app.include_router(auth.router, prefix="/api/auth", tags=["auth"])
 app.include_router(users.router, prefix="/api/users", tags=["users"])
 app.include_router(profile.router, prefix="/api/profile", tags=["profile"])
 app.include_router(admin.router, prefix="/api/admin", tags=["admin"])
+app.include_router(groups.router, prefix="/api/admin/groups", tags=["admin"])
 app.include_router(settings_router.router, prefix="/api/settings", tags=["settings"])
 app.include_router(services.router, prefix="/api/services", tags=["services"])
 # categories_proxy MUST be registered before documents_proxy —