feat: Implement staff invitation system with role-based permissions
- Add StaffInvitation model with token-based 7-day expiration
- Create invitation API endpoints (create, cancel, resend, accept, decline)
- Add permissions JSONField to User model for granular access control
- Implement frontend invite modal with role-specific permissions:
- Manager: can_invite_staff, can_manage_resources, can_manage_services,
can_view_reports, can_access_settings, can_refund_payments
- Staff: can_view_all_schedules, can_manage_own_appointments
- Add edit staff modal with permissions management and deactivate option
- Create AcceptInvitePage for invitation acceptance flow
- Add active/inactive staff separation with collapsible section
- Auto-create bookable resource when configured at invite time
- Remove Quick Add Appointment from dashboard
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Co-Authored-By: Claude <noreply@anthropic.com>
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