fix(scheduler,pipeline): fix WP auth attribute name and release slot on hard errors

- scheduler: use wordpress_app_password (not wordpress_password) so
  _fetch_wp_occupied_slots() can actually authenticate against the WP
  REST API — previously always returned empty set silently
- pipeline: release reserved publish slot when draft creation fails with
  a non-ValueError exception (e.g. WP API error), preventing permanently
  blocked slots on failed articles

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

backend/app/pipeline.py

@@ -391,6 +391,9 @@ def _process_article(article: dict[str, Any], stats: PipelineStats, settings: An
         except Exception as exc:
             logger.error("Draft-Erstellung für #%d fehlgeschlagen: %s", article_id, exc)
             update_article_status(article_id, "error", actor="pipeline", note=f"Draft-Fehler: {exc}")
+            # Release reserved slot so it's not permanently blocked by a failed article
+            from .scheduler import release_publish_slot
+            release_publish_slot(article_id)
             raise