fix(helm): replace broken nc -z in migrate job init container

BusyBox 1.37 nc -z outputs "punt!" and hangs. Use nc -w 2 with
stdin redirect instead, which correctly tests TCP connectivity.

backend/deploy/helm/templates/migrate-job.yaml

@@ -52,7 +52,7 @@ spec:
             - -c
             - |
               echo "Waiting for PostgreSQL..."
-              until nc -z "$DB_HOST" "$DB_PORT"; do
+              until nc -w 2 "$DB_HOST" "$DB_PORT" < /dev/null; do
                 sleep 3
               done
               echo "PostgreSQL is reachable"