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_document_intelligence.py

@@ -84,6 +84,35 @@ def test_document_intelligence_prefers_train_ticket_for_railway_e_ticket_invoice
     assert any(field.label == "金额" and field.value == "354元" for field in insight.fields)
 
 
+def test_document_intelligence_recovers_train_ticket_from_english_station_ocr_text() -> None:
+    insight = build_document_insight(
+        filename="2月20_武汉-上海.pdf",
+        summary=":26429165800002785705；:2026 05 18；Wuhan Shanghaihongqiao G458",
+        text=(
+            ":26429165800002785705\n"
+            ":2026 05 18\n"
+            "G458\n"
+            "Wuhan\n"
+            "Shanghaihongqiao\n"
+            "2026 02 20 07:55\n"
+            "06 01B\n"
+            ": 354.00\n"
+            "4201061987****1615\n"
+            ":6580061086021391007342026\n"
+            "12306 95306"
+        ),
+    )
+
+    assert insight.document_type == "train_ticket"
+    assert insight.document_type_label == "火车/高铁票"
+    assert insight.scene_code == "travel"
+    fields = {field.label: field.value for field in insight.fields}
+    assert fields["金额"] == "354元"
+    assert fields["列车出发时间"] == "2026-02-20 07:55"
+    assert fields["车次/航班"] == "G458"
+    assert fields["行程"] == "武汉-上海"
+
+
 def test_document_intelligence_labels_train_ticket_date_as_train_departure_time() -> None:
     insight = build_document_insight(
         filename="铁路电子客票.pdf",