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

drizzle/0003_lying_cassandra_nova.sql

@@ -0,0 +1,12 @@
+CREATE TABLE `materials` (
+	`associated_date` text NOT NULL,
+	`created_at` text NOT NULL,
+	`description` text NOT NULL,
+	`id` text PRIMARY KEY NOT NULL,
+	`project_id` text NOT NULL,
+	`status` text DEFAULT 'pending' NOT NULL,
+	`updated_at` text NOT NULL,
+	FOREIGN KEY (`project_id`) REFERENCES `projects`(`id`) ON UPDATE no action ON DELETE no action
+);
+--> statement-breakpoint
+CREATE INDEX `materials_project_id_idx` ON `materials` (`project_id`);