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/server/routes/chat.test.ts

@@ -44,12 +44,12 @@ async function patchConversationViaHandler(req: Request, db: Database): Promise<
   return h(req, db, MODE, LOG);
 }
 
-function seedModel(db: Database, providerId: string, modelName = "GPT-4o", modelId = "gpt-4o"): string {
+function seedModel(db: Database, providerId: string, modelName = "GPT-4o", externalId = "gpt-4o"): string {
   const result = createModel(
     db,
     {
       capabilities: ["text"],
-      modelId,
+      externalId,
       name: modelName,
       providerId,
     },
@@ -111,7 +111,7 @@ describe("聊天 API 路由", () => {
       }
     });
 
-    test("无可用模型时返回 400", async () => {
+    test("无可用模型时创建会话 modelId 为 null", async () => {
       const handle = createMigratedMemoryTestDatabase("chat-create-no-model");
       try {
         const db = handle.db;
@@ -123,9 +123,9 @@ describe("聊天 API 路由", () => {
           method: "POST",
         });
         const res = await createConversationViaHandler(req, db);
-        expect(res.status).toBe(400);
-        const body = (await res.json()) as { error: string };
-        expect(body.error).toContain("模型");
+        expect(res.status).toBe(201);
+        const body = (await res.json()) as { conversation: Conversation };
+        expect(body.conversation.modelId).toBeNull();
         handle.close();
       } finally {
         handle.cleanup();