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

src/server/routes/chat/send.ts

@@ -13,7 +13,7 @@ import {
   updateConversation,
   updateConversationTimestamp,
 } from "../../db/conversations";
-import { getModelWithProvider } from "../../db/models";
+import { getModelWithProvider, listModels } from "../../db/models";
 import { createApiError, jsonResponse } from "../../helpers";
 import { validateIdParam } from "../../middleware";
 
@@ -79,13 +79,23 @@ export async function handleSendChat(req: Request, db: Database, mode: RuntimeMo
 
   let model;
   try {
-    const result = getModelWithProvider(db, conversation.modelId);
+    let effectiveModelId = conversation.modelId;
+    if (!effectiveModelId) {
+      const fallback = listModels(db, { page: 1, pageSize: 1 });
+      const firstModel = fallback.items[0];
+      if (!firstModel) {
+        return jsonResponse(createApiError("没有可用的模型，请先配置模型", 400), { mode, status: 400 });
+      }
+      effectiveModelId = firstModel.id;
+    }
+
+    const result = getModelWithProvider(db, effectiveModelId);
     if ("error" in result) {
       return jsonResponse(createApiError(result.error, result.status), { mode, status: result.status });
     }
 
     const registry = buildProviderRegistry(db);
-    model = registry.languageModel(`${result.provider.id}:${result.model.modelId}`);
+    model = registry.languageModel(`${result.provider.id}:${result.model.externalId}`);
   } catch (e: unknown) {
     const msg = e instanceof Error ? e.message : String(e);
     return jsonResponse(createApiError(`模型初始化失败：${msg}`, 500), { mode, status: 500 });