Add PRODUCTION_CUTOVER.md and link from DEPLOYMENT.md

Cross-references the Civi-side cutover checklist from the deploy doc
so anyone deploying knows the CiviCRM prerequisites are tracked in a
single place.

DEPLOYMENT.md

@@ -3,6 +3,12 @@
 This app is built to deploy to **[Render](https://render.com)** as a Web
 Service via the included `render.yaml` blueprint.
 
+For the **CiviCRM-side** setup that must happen before this app can
+work (extension install, custom-field defaults, relationship types,
+option groups, post-deploy verification), see
+[`PRODUCTION_CUTOVER.md`](./PRODUCTION_CUTOVER.md). Keep that doc
+current — every Civi prerequisite we discover belongs there.
+
 ## Pre-flight checklist
 
 Before deploying, confirm: