docs: reference tubco maintenance policy in handbook

backend/workflows/templates/workflows/developer_handbook.html

@@ -673,6 +673,7 @@ lxc.mount.entry: /dev/null sys/module/apparmor/parameters/enabled none bind 0 0<
         <li><code>release/tubco-v1</code> is the frozen TUBCO customer branch.</li>
         <li>It should receive only approved bug fixes, security updates, and UI improvements.</li>
         <li>Do not deploy TUBCO from <code>develop</code> or <code>main</code>.</li>
+        <li>Keep TUBCO on the old customer database schema and review <code>docs/TUBCO_MAINTENANCE_POLICY.md</code> before backporting fixes.</li>
       </ul>
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