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/db/projects.test.ts

@@ -132,16 +132,13 @@ describe("项目数据访问层", () => {
       const result = archiveProject(db, id, createNoopLogger());
       expect("error" in result).toBe(false);
 
-      const archived = (result as { project: { archivedAt: null | string; status: string } }).project;
+      const archived = (result as { project: { status: string } }).project;
       expect(archived.status).toBe("archived");
-      expect(archived.archivedAt).not.toBeNull();
 
-      const row = db.query("SELECT status, archived_at FROM projects WHERE id = ?").get(id) as {
-        archived_at: null | string;
+      const row = db.query("SELECT status FROM projects WHERE id = ?").get(id) as {
         status: string;
       };
       expect(row.status).toBe("archived");
-      expect(row.archived_at).not.toBeNull();
     });
   });
 
@@ -165,9 +162,8 @@ describe("项目数据访问层", () => {
       const result = restoreProject(db, id, createNoopLogger());
       expect("error" in result).toBe(false);
 
-      const restored = (result as { project: { archivedAt: null | string; status: string } }).project;
+      const restored = (result as { project: { status: string } }).project;
       expect(restored.status).toBe("active");
-      expect(restored.archivedAt).toBeNull();
     });
   });