fix(search): split markdown into dedicated queue consumer to prevent pipeline stall

대형 PDF split 변환(5210 ≈ 40분 실측)이 단일 consume_queue 코루틴을 점유해
extract/classify/embed/chunk 등 전 파이프라인을 stall 시키던 문제 제거.

- consume_markdown_queue 신규 — markdown 전용 scheduler job (id=markdown_consumer)
- consume_queue 는 MAIN_QUEUE_STAGES (markdown 제외) 만 처리
- _process_stage / _load_workers 헬퍼로 per-stage 로직 공유
- reset_stale_items(stages, threshold_minutes) 파라미터화: main=10min(markdown 제외),
  markdown=MARKDOWN_STALE_MINUTES(기본 120). marker_worker 는 heartbeat 미기록이라
  40분 변환을 10분 stale 로 오인하던 함정 차단
- enqueue flow (classify -> embed,chunk,markdown) 불변

STT/deep_summary 분리 + GPU 동시성 튜닝은 out of scope (follow-up).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/main.py

@@ -51,7 +51,7 @@ async def lifespan(app: FastAPI):
     from workers.law_monitor import run as law_monitor_run
     from workers.mailplus_archive import run as mailplus_run
     from workers.news_collector import run as news_collector_run
-    from workers.queue_consumer import consume_queue
+    from workers.queue_consumer import consume_queue, consume_markdown_queue
     from workers.study_queue_consumer import consume_study_queue
     from workers.study_session_queue_consumer import consume_study_session_queue
     from workers.study_question_embed_worker import (
@@ -77,6 +77,10 @@ async def lifespan(app: FastAPI):
     scheduler = AsyncIOScheduler(timezone="Asia/Seoul")
     # 상시 실행
     scheduler.add_job(consume_queue, "interval", minutes=1, id="queue_consumer")
+    # PR-DocSrv-Markdown-Consumer-Split-1: markdown(marker) 전용 consumer.
+    # 대형 PDF split 변환(수십 분)이 메인 consume_queue 를 점유해 전 파이프라인을
+    # stall 시키던 문제 제거. max_instances=1(기본) 으로 동시 marker 변환 2건은 방지.
+    scheduler.add_job(consume_markdown_queue, "interval", minutes=1, id="markdown_consumer")
     scheduler.add_job(watch_inbox, "interval", minutes=5, id="file_watcher")
     scheduler.add_job(cleanup_orphan_uploads, "interval", minutes=10, id="upload_cleanup")
     # PR-4: study_questions 자동 임베딩 (status='none/failed/stale' 행을 batch=10 처리).