IT Assets: add role-based auth system and portal user sync

Auth:
- Fix local login (was redirecting to portal; now authenticates AdminUser directly)
- Portal SSO still takes priority in production via nginx headers

Role system (admin | editor | readonly):
- New app/utils/decorators.py with editor_required and admin_required decorators
- All write routes protected with editor_required (create/edit/delete/import/mask)
- Settings user management protected with admin_required
- Sidebar hides write-only links for readonly users
- Dashboard quick actions and list page buttons hidden for readonly

Settings page:
- Role colour badges (admin=red, editor=blue, readonly=grey)
- Inline role changer per user (dropdown auto-submit)
- Reset password modal per user
- Delete user button with confirmation
- Add user form includes role selector with legend

Portal user sync:
- New /internal/sync-user endpoint receives user pre-creation from portal
- INTERNAL_SYNC_SECRET added to config
- portal/config.py: added internal_url for itassets app so _sync_user_to_app works

IT_asset_management/app/templates/dashboard/index.html

@@ -102,6 +102,7 @@
 </div>
 
 <!-- ── Quick Actions ──────────────────────────────────────────── -->
+{% if current_user.is_editor %}
 <div class="row g-3 mb-4">
   <div class="col-12">
     <div class="card border-0 shadow-sm">
@@ -123,6 +124,7 @@
     </div>
   </div>
 </div>
+{% endif %}
 
 <!-- ── Recent Assignments ─────────────────────────────────────── -->
 <div class="card border-0 shadow-sm">