feat: 新增配送合同附件安全上传功能

- 增加合同专用 PDF 上传、鉴权预览和失败清理接口
- 支持草稿附件替换移除、并发保护和启用完整性校验
- 修复循环模板引用导致文件选择器无法打开的问题
- 补充专项测试、中文项目文档和操作日志

frontend/platform_admin/package.json

@@ -16,6 +16,7 @@
     "resource-pages:check": "node scripts/check-resource-pages.mjs",
     "account-roles:check": "node scripts/check-account-role-presentation.mjs",
     "avatar-retry:check": "node scripts/check-avatar-upload-cache.mjs",
+    "contract-attachment:check": "node scripts/check-contract-attachment-control.mjs",
     "staff-organization:check": "node scripts/check-staff-organization-linkage.mjs",
     "staff-relations:check": "node scripts/check-staff-relation-policy.mjs",
     "user-address-display:check": "node scripts/check-user-address-relation-display.mjs",