Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/scripts/upgradeable/patch-apply.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+DIRNAME="$(dirname -- "${BASH_SOURCE[0]}")"
+PATCH="$DIRNAME/upgradeable.patch"
+
+error() {
+  echo Error: "$*" >&2
+  exit 1
+}
+
+if ! git diff-files --quiet ":!$PATCH" || ! git diff-index --quiet HEAD ":!$PATCH"; then
+  error "Repository must have no staged or unstaged changes"
+fi
+
+if ! git apply -3 "$PATCH"; then
+  error "Fix conflicts and run $DIRNAME/patch-save.sh"
+fi