feat(inbox): 素材持久化 CRUD — 数据库表 + API + 前端接入

- 新增 materials 表（id/projectId/description/associatedDate/status/createdAt/updatedAt）
- 新增 4 个后端 API 路由（list/create/get/delete）+ 13 个测试
- 新增 use-materials hooks（TanStack Query）
- 收集箱页面重构为三层架构（InboxPage + MaterialSidebar + MaterialDetailPanel）
- MaterialCard: Popconfirm 删除确认 + 粗粒度时间格式
- MaterialContent: 展示状态标签 + createdAt
- 更新开发文档 backend.md / frontend.md

src/web/features/inbox/components/MaterialList.tsx

@@ -1,11 +1,12 @@
 import { PlusOutlined } from "@ant-design/icons";
-import { Button, Empty } from "antd";
+import { Button, Empty, Spin } from "antd";
 
 import type { Material } from "../types";
 
 import { MaterialCard } from "./MaterialCard";
 
 interface MaterialListProps {
+  loading: boolean;
   materials: readonly Material[];
   onAddClick: () => void;
   onDelete: (id: string) => void;
@@ -13,14 +14,16 @@ interface MaterialListProps {
   selectedId: null | string;
 }
 
-export function MaterialList({ materials, onAddClick, onDelete, onSelect, selectedId }: MaterialListProps) {
+export function MaterialList({ loading, materials, onAddClick, onDelete, onSelect, selectedId }: MaterialListProps) {
   return (
     <div className="app-inbox-sidebar">
       <Button block icon={<PlusOutlined />} onClick={onAddClick} type="primary">
         新增素材
       </Button>
       <div className="app-inbox-list">
-        {materials.length === 0 ? (
+        {loading ? (
+          <Spin />
+        ) : materials.length === 0 ? (
           <Empty description="暂无素材" image={Empty.PRESENTED_IMAGE_SIMPLE} />
         ) : (
           materials.map((material) => (