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/Service/AlertService.php

@@ -15,7 +15,7 @@ use OCP\IGroupManager;
 use OCP\Notification\IManager;
 
 /** Notifies administrators about failed runs and locations without a recent successful backup. */
-class AlertService {
+final class AlertService {
 	public const STALE_AFTER = 2 * 86400;
 
 	public function __construct(