renovate: Enable monthly lock file maintenance

Author: Turbo87

.github/renovate.json5

@@ -1,6 +1,9 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": [
+    // Refresh lock files on the first day of each month
+    // (still gated by dashboard approval for now)
+    ":maintainLockFilesMonthly",
     // Pin GitHub Actions to their commit SHA digests, resolving floating tags
     // (e.g. `v4`) to the full SemVer version (e.g. `v4.1.2`)
     "helpers:pinGitHubActionDigestsToSemver"