fix: exhibit/bulk에서 concurrent DB 세션 충돌 수정

asyncio.gather로 동시에 실행되는 exhibit 항목들이 단일 DB 세션을
공유하면서 "concurrent operations are not permitted" 에러 발생.

각 _fetch_one 태스크가 AsyncSessionLocal()로 독립 세션 사용하도록 수정.

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

app/api/v1/endpoints/filings.py

@@ -10,7 +10,7 @@ from typing import List, Optional
 from fastapi import APIRouter, Depends, HTTPException, Query, Response
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.core.database import get_db
+from app.core.database import get_db, AsyncSessionLocal
 from app.schemas.filing import (
     BulkExhibitItem,
     BulkExhibitRequest,
@@ -290,27 +290,29 @@ async def get_exhibit_bulk(
     async def _fetch_one(item: dict) -> BulkExhibitItem:
         accession_number = item.get("accession_number", "")
         exhibit_type = item.get("exhibit_type", "EX-99.1")
-        try:
-            result = await asyncio.wait_for(
-                sec_filings_service.get_exhibit_content(db, accession_number, exhibit_type),
-                timeout=30,
-            )
-            return BulkExhibitItem(
-                accession_number=accession_number,
-                exhibit_type=exhibit_type,
-                success=True,
-                content=result["content"],
-                content_type=result.get("content_type"),
-                filename=result.get("filename"),
-                url=result.get("url"),
-            )
-        except Exception as e:
-            return BulkExhibitItem(
-                accession_number=accession_number,
-                exhibit_type=exhibit_type,
-                success=False,
-                error=str(e),
-            )
+        # Each concurrent call gets its own DB session to avoid session contention
+        async with AsyncSessionLocal() as session:
+            try:
+                result = await asyncio.wait_for(
+                    sec_filings_service.get_exhibit_content(session, accession_number, exhibit_type),
+                    timeout=30,
+                )
+                return BulkExhibitItem(
+                    accession_number=accession_number,
+                    exhibit_type=exhibit_type,
+                    success=True,
+                    content=result["content"],
+                    content_type=result.get("content_type"),
+                    filename=result.get("filename"),
+                    url=result.get("url"),
+                )
+            except Exception as e:
+                return BulkExhibitItem(
+                    accession_number=accession_number,
+                    exhibit_type=exhibit_type,
+                    success=False,
+                    error=str(e),
+                )
 
     raw = await asyncio.gather(*[_fetch_one(item) for item in request.items], return_exceptions=True)
     results: List[BulkExhibitItem] = []