fix: check registrar's /health endpoint, not /heartbeat

The latter endpoint doesn't exist on registrar. It 404s.

This was causing registrar's healthcheck to fail, but the
failure was passing silently due to a bug in the reporting
logic of check.sh (fixed in an adjacent commit).

Author: kdmccormick

check.sh

@@ -56,7 +56,7 @@ run_check() {
 if should_check registrar; then
     echo "Checking Registrar heartbeat:"
     run_check registrar_heartbeat registrar \
-        "curl --fail -L http://localhost:18734/heartbeat"
+        "curl --fail -L http://localhost:18734/health"
 fi
 
 if should_check lms; then