fix: persist uploads on Render disk and show missing file notice

Co-authored-by: Cursor <cursoragent@cursor.com>

render.yaml

@@ -5,6 +5,11 @@ services:
     rootDir: backend
     buildCommand: npm install --include=dev && (npx prisma migrate deploy || true) && npx prisma db push && npx prisma generate && npm run build
     startCommand: npx prisma db push && npm start
+    # 첨부 파일이 재배포 후에도 유지되도록 영구 디스크 (Render 유료 플랜 필요)
+    disk:
+      name: uploads-data
+      mountPath: /opt/render/project/src/backend/uploads
+      sizeGB: 1
     envVars:
       - key: DATABASE_URL
         sync: false   # Render 대시보드에서 직접 입력
@@ -17,7 +22,7 @@ services:
       - key: JWT_EXPIRES_IN
         value: 7d
       - key: UPLOAD_DIR
-        value: ./uploads
+        value: /opt/render/project/src/backend/uploads
       - key: MAX_FILE_SIZE_MB
         value: 20
       - key: NODE_ENV