Implement a full permissions system replacing simple admin/user roles:
Backend:
- Add Group model with many-to-many user relationship
- Add 50+ granular permissions (resource:action pattern)
- Create default groups: Administrators, Operators, Viewers
- Add permission-checking dependencies for route protection
- Add groups API endpoints (CRUD, user assignment)
- Add change password endpoint for users
- Update backup/restore to include groups
- Migrate existing users to groups on startup
Frontend:
- Add GroupsPage for managing groups and permissions
- Add permission helpers to AuthContext (hasPermission, hasAnyPermission)
- Add PermissionRoute component for protected routes
- Disable buttons/features based on permissions (with tooltips)
- Add change password modal in sidebar for all users
- Add forgot password info modal on login page
- Show user groups in UsersPage with group assignment
Testing:
- Add integration tests for groups API
- Add tests for user-group assignments
- Add tests for change password endpoint
- Seed default groups in test fixtures
Closes#28#161
