Fix: Restore status 4 (existing langlink) in translate completion check

Nik Gkountasrequested to merge
fix/restore-translate-langlink-status-4 into main

Reverts the status-4 removal from commit 13072adf. The translate path again reports status 4 when the source article already has a langlinked translation on the target wiki, and that status counts as completed.

  • getTranslateCompletionStatus (server/routes/tasks.js) again fetches checkIfArticleExistsByLanglink and getCtTranslationStatus in parallel and returns 4 when the article exists but was not published via CT. JSDoc restored to the 0-4 scale here and in getEssentialArticleCompletionStatus.
  • completedStatuses restored to [3, 4] for translate-policy-page and create-essential-article in client/src/registry/essential-tasks.js.
  • Restored the JSDoc for checkIfArticleExistsByLanglink in server/services/mediawiki.js (the method body was never removed, only its doc comment and its call site).

Left untouched from 13072adf: the new DELETE /api/tasks/:taskKey QA reset route and the Wikidata-sitelink picker filter in useEssentialArticleSource (later work builds on it).

Co-Authored-By: Claude Opus 4.8 Bug: T428504

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