Detect dead links as permanent errors, fix last-episode extraction race

Dead link detection:
- Mega-Web: parse hoster error messages (hosterNotAvailable, etc.) from HTML
  and throw specific error instead of returning null
- MegaDebridClient: stop retrying on permanent hoster errors
- download-manager: isPermanentLinkError() immediately fails items with dead
  links instead of retrying forever

Extraction race condition:
- package_done cleanup policy checked if all items were "completed" (downloaded)
  but not if they were "extracted" — removing the package before the last
  episode could be extracted
- Both applyCompletedCleanupPolicy and applyPackageDoneCleanup now guard
  against premature removal when autoExtract is enabled

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/debrid.ts

@@ -505,6 +505,10 @@ class MegaDebridClient {
       if (!lastError) {
         lastError = web ? "Mega-Web Antwort ohne Download-Link" : "Mega-Web Antwort leer";
       }
+      // Don't retry permanent hoster errors (dead link, file removed, etc.)
+      if (/permanent ungültig|hosternotavailable|file.?not.?found|file.?unavailable|link.?is.?dead/i.test(lastError)) {
+        break;
+      }
       if (attempt < REQUEST_RETRIES) {
         await sleepWithSignal(retryDelay(attempt), signal);
       }