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/Service/TargetService.php

@@ -152,7 +152,11 @@ final class TargetService {
 	}
 
 	public function get(string $nameOrId): Target {
-		return ctype_digit($nameOrId) ? $this->mapper->find((int)$nameOrId) : $this->mapper->findByName($nameOrId);
+		try {
+			return ctype_digit($nameOrId) ? $this->mapper->find((int)$nameOrId) : $this->mapper->findByName($nameOrId);
+		} catch (DoesNotExistException) {
+			throw new \InvalidArgumentException("No such location: $nameOrId");
+		}
 	}
 
 	/** Size limit for the data stored on this location (null = none). */