IT Assets: add portal sync button + wire env vars in start-dev.sh

- Portal: add /api/internal/itassets-users endpoint (returns all portal users
  with itassets access + their effective role)
- IT Assets: add PORTAL_INTERNAL_URL config key
- IT Assets: add POST /settings/sync-from-portal route (admin only) — pulls
  users from the portal API and upserts them locally
- Settings page: 'Sync from Portal' button next to the users table header
- start-dev.sh: pass INTERNAL_SYNC_SECRET and ITASSETS_INTERNAL_URL to portal;
  pass PORTAL_INTERNAL_URL and INTERNAL_SYNC_SECRET to itassets
- Also includes 'Back to Portal' icon button added to sidebar footer

IT_asset_management/app/templates/base.html

@@ -190,7 +190,10 @@
     <span class="badge ms-1 {% if current_user.role == 'admin' %}bg-danger{% elif current_user.role == 'editor' %}bg-primary{% else %}bg-secondary{% endif %}" style="font-size:.65rem;">
       {{ current_user.role }}
     </span>
-    <a href="{{ url_for('auth.logout') }}" class="ms-2 text-warning text-decoration-none">
+    <a href="/" class="ms-2 text-info text-decoration-none" title="Back to Portal">
+      <i class="bi bi-house-door"></i>
+    </a>
+    <a href="{{ url_for('auth.logout') }}" class="ms-1 text-warning text-decoration-none" title="Sign out">
       <i class="bi bi-box-arrow-right"></i>
     </a>
   </div>