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/Command/KeyConfirm.php

@@ -14,7 +14,7 @@ use Symfony\Component\Console\Input\InputOption;
 use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\Console\Question\Question;
 
-class KeyConfirm extends Command {
+final class KeyConfirm extends Command {
 	public const STATEMENT = 'I have downloaded the recovery kit and stored it safely, outside this server. '
 		. 'I understand that keeping these keys safe is my own responsibility and that backups cannot be restored without them.';
 	public const CONFIRMATION = 'Yes, I confirm';