feat(server): 票据文件夹资产缓存与文档预览统一生成

- 新增 document_preview 模块，DocumentPreviewAssets 统一处理 data URL 解码、pdftoppm PNG 预览生成（poppler-data 编码）、renderer_id 标识
- receipt_folder 服务复用预览生成，缓存票据资产并提供清理；删除票据时保留已关联报销单的附件副本
- document_intelligence 新增票据预览/资产缓存接入与字段提取增强；ocr 抽取复用预览工具，附件分析/文档/操作/展示四个子模块同步适配
- receipt_folder 端点补充资产缓存头，补/扩 document_intelligence、ocr_endpoints、ocr_service、receipt_folder_service、reimbursement_endpoints 测试，新增 attachment_analysis 回归测试

server/src/app/api/v1/endpoints/receipt_folder.py

@@ -92,7 +92,7 @@ def preview_receipt(receipt_id: str, current_user: CurrentUser) -> FileResponse:
         file_path, media_type, file_name = ReceiptFolderService().resolve_preview(receipt_id, current_user)
     except FileNotFoundError as exc:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Receipt preview not found") from exc
-    return FileResponse(file_path, media_type=media_type, filename=file_name)
+    return FileResponse(file_path, media_type=media_type, filename=file_name, headers={"Cache-Control": "no-store"})
 
 
 @router.get(