docs: phase 2 configuration and the reaper's containment

FREE_INSTANCE_REAP_AFTER is set only in docker-compose.site.yml, so a
self-hosted deployment can never reap. Admin and server must carry the
same value: one names the deletion date in warnings, the other acts on it.

Records that admin now has a second control-plane write path, cloudprov,
and that deletion lives in the control plane because that is where the
knowledge of what an instance is made of belongs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/server-deploy.yml

@@ -43,6 +43,7 @@ jobs:
                   docker build \
                     --build-arg NEXT_PUBLIC_SITE_API="${{ vars.SITE_API_URL }}" \
                     --build-arg NEXT_PUBLIC_CONTACT_EMAIL="support@hostxtra.co.uk" \
+                    --build-arg NEXT_PUBLIC_ADMIN_API_URL="${{ vars.ADMIN_API_URL }}" \
                     -t "$IMAGE" \
                     -f site/Dockerfile site/
                   docker push "$IMAGE"