Ask before installing user_migration; add the beta how-to

backup:user:backup and backup:user:restore now ask the administrator before
installing or enabling user_migration (or take --install-user-migration), so
nothing changes on the server without consent. The restore warns that
user_migration imports only its allowlisted app settings.

Also: a clear error for unknown locations instead of a raw database message;
prune reports the number of affected users, not the number of kept ones; and
--user-last rejects values that are not whole numbers. Adds docs/HOWTO.md and
updates the README and PLAN.md for the beta.

lib/Command/Prune.php

@@ -48,8 +48,9 @@ final class Prune extends Command {
 				continue;
 			}
 			if ($ru['forgotten'] !== []) {
+				$affected = count(array_unique(array_map(static fn (string $path): string => explode('/', $path)[1] ?? '', $ru['forgotten'])));
 				$output->writeln(sprintf('<info>%s</info>: %s %d user export(s) for %d user(s)',
-					$t->getName(), $dry ? 'would forget' : 'forgot', count($ru['forgotten']), count($ru['kept'])));
+					$t->getName(), $dry ? 'would forget' : 'forgot', count($ru['forgotten']), $affected));
 			}
 			try {
 				$r = $this->prune->apply($t, $dry);