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/PruneService.php

@@ -20,7 +20,7 @@ use OCP\Lock\LockedException;
  * prunes data no remaining snapshot uses. Never runs on append-only locations, never while a
  * backup to the same location is running (the location lock is shared with BackupService).
  */
-class PruneService {
+final class PruneService {
 	public function __construct(
 		private TargetService $targets,
 		private RunMapper $runs,