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/tests/test_ocr_service.py

@@ -101,6 +101,55 @@ print("__OCR_JSON__=" + json.dumps(payload, ensure_ascii=False))
     assert skipped.warnings == ["当前仅支持图片和 PDF 文件进行 OCR。"]
 
 
+def test_ocr_service_recovers_image_text_from_worker_ocr_text(
+    monkeypatch,
+    tmp_path: Path,
+) -> None:
+    def fake_invoke_worker(
+        self,
+        *,
+        python_bin: str,
+        worker_path: str,
+        input_paths: list[Path],
+    ) -> dict:
+        return {
+            "engine": "paddleocr_mobile",
+            "model": "PP-OCRv5_mobile",
+            "documents": [
+                {
+                    "input_path": str(input_paths[0]),
+                    "engine": "paddleocr_mobile",
+                    "model": "PP-OCRv5_mobile",
+                    "ocr_text": "铁路电子客票 武汉-上海 2026 02 20 07:55 G458 : 354.00 12306 95306",
+                    "avg_score": 0.92,
+                    "line_count": 0,
+                    "page_count": 1,
+                    "warnings": [],
+                    "lines": [],
+                }
+            ],
+        }
+
+    monkeypatch.setenv("STORAGE_ROOT_DIR", str(tmp_path / "storage"))
+    monkeypatch.setattr(OcrService, "_resolve_python_bin", lambda self: "python")
+    monkeypatch.setattr(OcrService, "_resolve_worker_path", lambda self: "worker.py")
+    monkeypatch.setattr(OcrService, "_invoke_worker", fake_invoke_worker)
+    OcrService._result_cache.clear()
+    get_settings.cache_clear()
+    try:
+        result = OcrService().recognize_files([("train-ticket.png", b"fake-train-image", "image/png")])
+    finally:
+        OcrService._result_cache.clear()
+        get_settings.cache_clear()
+
+    recognized = result.documents[0]
+    assert "铁路电子客票" in recognized.text
+    assert recognized.document_type == "train_ticket"
+    assert any(field.label == "列车出发时间" and field.value == "2026-02-20 07:55" for field in recognized.document_fields)
+    assert any(field.label == "车次/航班" and field.value == "G458" for field in recognized.document_fields)
+    assert any(field.label == "金额" and field.value == "354元" for field in recognized.document_fields)
+
+
 def test_ocr_service_passes_configured_device_to_worker(
     monkeypatch,
     tmp_path: Path,