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/services/expense_claim_attachment_operations.py

@@ -32,6 +32,7 @@ from app.services.agent_asset_rule_library import AgentAssetRuleLibraryManager
 from app.services.agent_asset_spreadsheet import RISK_RULES_LIBRARY
 from app.services.agent_foundation import AgentFoundationService
 from app.services.audit import AuditLogService
+from app.services.document_preview import DocumentPreviewAssets
 from app.services.document_intelligence import build_document_insight
 from app.services.expense_claim_access_policy import ExpenseClaimAccessPolicy
 from app.services.expense_claim_attachment_presentation import ExpenseClaimAttachmentPresentation
@@ -238,6 +239,7 @@ class ExpenseClaimAttachmentOperationsMixin:
             "preview_storage_key": str(preview_meta["preview_storage_key"]),
             "preview_media_type": str(preview_meta["preview_media_type"]),
             "preview_file_name": str(preview_meta["preview_file_name"]),
+            "preview_rendered_with": str(preview_meta.get("preview_rendered_with") or ""),
             "analysis": attachment_analysis,
             "document_info": document_info,
             "requirement_check": requirement_check,
@@ -673,6 +675,60 @@ class ExpenseClaimAttachmentOperationsMixin:
         self._attachment_storage.write_meta(file_path, metadata)
         return metadata
 
+    def _refresh_pdf_attachment_preview_meta_if_needed(
+        self,
+        *,
+        file_path: Path,
+        metadata: dict[str, Any],
+    ) -> dict[str, Any]:
+        if not metadata:
+            return metadata
+
+        media_type = str(
+            metadata.get("media_type")
+            or self._attachment_presentation.resolve_media_type(file_path.name)
+        ).strip()
+        if media_type != "application/pdf":
+            return metadata
+
+        preview_storage_key = str(metadata.get("preview_storage_key") or "").strip()
+        preview_path = self._attachment_storage.resolve_path(preview_storage_key) if preview_storage_key else None
+        if (
+            preview_path is not None
+            and preview_path.exists()
+            and str(metadata.get("preview_kind") or "").strip() == "image"
+            and str(metadata.get("preview_media_type") or "").strip() == DocumentPreviewAssets.PDF_PREVIEW_MEDIA_TYPE
+            and str(metadata.get("preview_rendered_with") or "").strip() == DocumentPreviewAssets.PDF_RENDERER_ID
+        ):
+            return metadata
+
+        preview_name = str(metadata.get("preview_file_name") or "").strip()
+        if not preview_name or not preview_name.lower().endswith(DocumentPreviewAssets.PDF_PREVIEW_SUFFIX):
+            preview_name = f"{file_path.stem}.preview{DocumentPreviewAssets.PDF_PREVIEW_SUFFIX}"
+        preview_path = file_path.parent / preview_name
+
+        try:
+            DocumentPreviewAssets.render_pdf_first_page(
+                pdf_path=file_path,
+                preview_path=preview_path,
+                timeout_seconds=OcrService(self.db).settings.ocr_timeout_seconds,
+            )
+        except Exception:
+            return metadata
+
+        metadata.update(
+            {
+                "previewable": True,
+                "preview_kind": "image",
+                "preview_storage_key": self._attachment_storage.to_storage_key(preview_path),
+                "preview_media_type": DocumentPreviewAssets.PDF_PREVIEW_MEDIA_TYPE,
+                "preview_file_name": preview_path.name,
+                "preview_rendered_with": DocumentPreviewAssets.PDF_RENDERER_ID,
+            }
+        )
+        self._attachment_storage.write_meta(file_path, metadata)
+        return metadata
+
     def _resolve_item_attachment_preview_content(self, item: ExpenseClaimItem) -> tuple[Path, str, str]:
         file_path, media_type, filename = self._resolve_item_attachment_content(item)
         metadata = self._attachment_storage.read_meta(file_path)
@@ -681,6 +737,10 @@ class ExpenseClaimAttachmentOperationsMixin:
             metadata=metadata,
             item=item,
         )
+        metadata = self._refresh_pdf_attachment_preview_meta_if_needed(
+            file_path=file_path,
+            metadata=metadata,
+        )
         preview_storage_key = str(metadata.get("preview_storage_key") or "").strip()
         preview_file_name = str(metadata.get("preview_file_name") or "").strip()
         preview_media_type = str(metadata.get("preview_media_type") or "").strip()