feat(pipeline): add two-stage article quality gate (min word count)

Stage 1 (before OpenAI rewrite): reject if raw content < pipeline_min_words_raw (default 120)
Stage 2 (after rewrite): reject if rewritten text < pipeline_min_words_rewritten (default 150)

Both stages set status='error' with a descriptive note and skip WP draft creation.
The reserved publish slot is released so it stays available for the next article.
Quality rejections don't abort the pipeline — processing continues with the next article.

New config settings (overridable via .env):
  PIPELINE_MIN_WORDS_RAW=120
  PIPELINE_MIN_WORDS_REWRITTEN=150

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

backend/app/scheduler.py

@@ -165,6 +165,15 @@ def _find_next_free_slot(
     return tomorrow, _preferred_hours()[0] if _preferred_hours() else 9
 
 
+def release_publish_slot(article_id: int) -> None:
+    """Clear a previously reserved slot (e.g. when article is rejected after slot assignment)."""
+    with get_conn() as conn:
+        conn.execute(
+            "UPDATE articles SET scheduled_publish_at = NULL WHERE id = ?",
+            (article_id,),
+        )
+
+
 def suggest_publish_slot() -> str:
     """Return a suggested publish datetime string (CET) for the next free slot."""
     wp_occupied = _fetch_wp_occupied_slots()