refactor(db): 统一数据库 schema — 软删除、命名规范、约束标准化

- 全表新增 deleted_at 列，统一软删除替代硬删除+archived_at
- models.model_id 重命名为 external_id，消除语义混淆
- conversations.model_id 改为可空（模型为建议而非绑定）
- messages 新增 updated_at，移除 CASCADE 改为 DAO 层级联
- 移除 DB 层 UNIQUE 约束，改为应用层检查（配合软删除）
- 新增 helpers.ts（baseColumns + 构造层防御）、ESLint 规则、契约测试
- 迁移 0004 补全 CHECK 约束（providers.type/materials.status/messages.role）
- DAO 层全面重写：级联软删除、应用层唯一、provider 删除保护
- 路由/前端/测试全量适配 externalId 重命名及类型变更

tests/web/routes/workbench.test.tsx

@@ -6,7 +6,6 @@ import { App } from "../../../src/web/app";
 import { renderWithProviders } from "../test-utils";
 
 const MOCK_PROJECT = {
-  archivedAt: null,
   createdAt: "2024-01-01T00:00:00.000Z",
   description: "测试项目",
   id: "test-project-id",
@@ -15,7 +14,7 @@ const MOCK_PROJECT = {
   updatedAt: "2024-01-01T00:00:00.000Z",
 };
 
-function createMockHandler(overrides?: { archivedAt?: string; status?: "active" | "archived" }) {
+function createMockHandler(overrides?: { status?: "active" | "archived" }) {
   const project = { ...MOCK_PROJECT, ...overrides };
   const handler = (input: RequestInfo | URL) => {
     const url = input instanceof Request ? input.url : typeof input === "string" ? input : input.toString();
@@ -92,7 +91,7 @@ describe("Workbench 路由", () => {
   });
 
   test("archived 项目显示不可访问", async () => {
-    createMockHandler({ archivedAt: "2024-06-01T00:00:00.000Z", status: "archived" });
+    createMockHandler({ status: "archived" });
 
     renderWithProviders(createElement(App), {
       initialRoute: `/workbench/${MOCK_PROJECT.id}`,