docs: registry is primary image source, ghcr is backup mirror

.github/workflows/release.yml

@@ -1,4 +1,7 @@
-name: Release
+name: Release (GHCR mirror)
+
+# Temporary backup mirror only. Production pulls from registry.ginnoir.com via
+# .gitea/workflows/release.yml — do not deploy from ghcr.io directly.
 
 on:
   push: