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/api/documents.py

@@ -17,7 +17,7 @@ from core.config import settings
 from core.database import get_session
 from core.utils import file_hash
 from models.document import Document
-from models.queue import ProcessingQueue
+from models.queue import ProcessingQueue, enqueue_stage
 from models.user import User
 
 router = APIRouter()
@@ -473,11 +473,7 @@ async def upload_document(
     await session.flush()
 
     # 처리 큐 등록
-    session.add(ProcessingQueue(
-        document_id=doc.id,
-        stage="extract",
-        status="pending",
-    ))
+    await enqueue_stage(session, doc.id, "extract")
     await session.commit()
 
     return DocumentResponse.model_validate(doc)

app/api/memos.py

@@ -13,7 +13,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from core.auth import get_current_user
 from core.database import get_session
 from models.document import Document
-from models.queue import ProcessingQueue
+from models.queue import ProcessingQueue, enqueue_stage
 from models.user import User
 
 router = APIRouter()
@@ -57,11 +57,7 @@ async def _enqueue_ai_stages(session: AsyncSession, document_id: int):
         )
     )
     for stage in stages:
-        session.add(ProcessingQueue(
-            document_id=document_id,
-            stage=stage,
-            status="pending",
-        ))
+        await enqueue_stage(session, document_id, stage)
 
 
 # ─── 스키마 ───