fix(queue): enqueue 경로 중복 방어 — partial unique index + 중앙 enqueue_stage 함수

기존 UNIQUE(document_id, stage, status)는 pending+processing 동시 존재를
허용해서 stale 복구 시 충돌 발생. 2-layer 방어로 근본 차단:

1) DB: partial unique index uq_queue_active — 활성 행(pending/processing)은
   (document_id, stage)당 최대 1개만 허용
2) App: enqueue_stage() 중앙 함수 — INSERT ON CONFLICT DO NOTHING으로
   모든 9개 경로의 check-then-insert TOCTOU race 제거

migration 117은 guard check 포함 — 활성 중복이 남아있으면 RAISE EXCEPTION
으로 중단, 수동 정리 유도.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/workers/queue_consumer.py

@@ -8,7 +8,7 @@ from sqlalchemy.orm import aliased
 
 from core.database import async_session
 from core.utils import setup_logger
-from models.queue import ProcessingQueue
+from models.queue import ProcessingQueue, enqueue_stage
 
 logger = setup_logger("queue_consumer")
 
@@ -103,21 +103,7 @@ async def enqueue_next_stage(document_id: int, current_stage: str):
 
     async with async_session() as session:
         for next_stage in stages:
-            existing = await session.execute(
-                select(ProcessingQueue).where(
-                    ProcessingQueue.document_id == document_id,
-                    ProcessingQueue.stage == next_stage,
-                    ProcessingQueue.status.in_(["pending", "processing"]),
-                )
-            )
-            if existing.scalar_one_or_none():
-                continue
-
-            session.add(ProcessingQueue(
-                document_id=document_id,
-                stage=next_stage,
-                status="pending",
-            ))
+            await enqueue_stage(session, document_id, next_stage)
         await session.commit()