fix: chown migrated legacy storage entries to dokku

`SaveEntry` and `touchMigrationFlag` wrote files via `os.WriteFile` and `os.Create` and never chowned them, so the install-time legacy-mount migration produced root-owned `legacy-*.json` files in `/var/lib/dokku/data/storage-registry/entries/`. The dokku user that runs `ps:rebuild` could not read them and every rebuild on a 0.37.x to 0.38.0 upgrade failed with permission denied. The companion `repairRegistryOwnership` pass in `TriggerInstall` rewrites ownership across the whole registry tree so installs that already ran the buggy code are fixed on the next package upgrade, since the per-app migration flag would otherwise cause `MigrateLegacyMounts` to skip the broken files forever.

plugins/storage/entry.go

@@ -148,8 +148,11 @@ func SaveEntry(entry *Entry) error {
 		return fmt.Errorf("unable to encode storage entry %q: %w", entry.Name, err)
 	}
 
-	path := entryPath(entry.Name)
-	if err := os.WriteFile(path, data, 0600); err != nil {
+	if err := common.WriteBytesToFile(common.WriteBytesToFileInput{
+		Bytes:    data,
+		Filename: entryPath(entry.Name),
+		Mode:     0600,
+	}); err != nil {
 		return fmt.Errorf("unable to write storage entry %q: %w", entry.Name, err)
 	}
 	return nil