Wire Psalm into CI, fix a real bug it found in user-migration import

RepositoryImportSource::copyToFolder() called putContent() on the generic
return type of Node::get(), which doesn't declare that method -- only
File does. Existing-file overwrite during a per-user restore would have
fatally errored at runtime; added an instanceof check and a resource leak
fix (finally-close) while there. Also guarded getFileAsStream() against
fopen() returning false.

Everything else Psalm (strict level) currently flags is mechanical
(missing `final` on leaf classes -- applied everywhere safe) or pre-existing
array-shape/type gaps, captured in psalm-baseline.xml so CI fails only on
new issues from here on, not the whole backlog at once.

lib/BackgroundJob/BackupJob.php

@@ -22,7 +22,7 @@ use Psr\Log\LoggerInterface;
  * With system cron a step may take minutes; with webcron/AJAX cron it is kept short so it fits
  * in a normal web request (hosted Nextcloud).
  */
-class BackupJob extends TimedJob {
+final class BackupJob extends TimedJob {
 	public function __construct(
 		ITimeFactory $time,
 		private BackupService $backups,