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/models.test.tsx

@@ -32,9 +32,9 @@ const ENABLED_MODEL: Model = {
   capabilities: ["text", "reasoning"],
   contextLength: 128000,
   createdAt: "2024-01-01T00:00:00.000Z",
+  externalId: "gpt-4o",
   id: "m1",
   maxOutputTokens: 4096,
-  modelId: "gpt-4o",
   name: "GPT-4o",
   providerId: "pv1",
   updatedAt: "2024-01-01T00:00:00.000Z",
@@ -190,7 +190,7 @@ describe("ModelFormModal", () => {
 
     await waitFor(() =>
       expect(testModelConnection).toHaveBeenCalledWith({
-        modelId: "gpt-4o",
+        externalId: "gpt-4o",
         providerId: "pv1",
       }),
     );
@@ -231,9 +231,9 @@ const TEST_MODEL: Model = {
   capabilities: ["text"],
   contextLength: 128000,
   createdAt: "2024-01-01T00:00:00.000Z",
+  externalId: "gpt-4o",
   id: "m1",
   maxOutputTokens: 4096,
-  modelId: "gpt-4o",
   name: "GPT-4o",
   providerId: "pv1",
   updatedAt: "2024-01-01T00:00:00.000Z",
@@ -281,7 +281,7 @@ function createModelFetchMock() {
 
     if (url.pathname === "/api/models" && call.method === "GET") {
       const keyword = url.searchParams.get("keyword") ?? "";
-      const items = keyword ? models.filter((m) => `${m.name}${m.modelId}`.includes(keyword)) : models;
+      const items = keyword ? models.filter((m) => `${m.name}${m.externalId}`.includes(keyword)) : models;
       return jsonResponse({ items, page: 1, pageSize: 20, total: items.length });
     }