refactor: v2 重构 — 模块化拆分 + AI Prompt模板化 + 前端状态管理

P0 - AI交互层:
- ai/prompt.js: 模块化 Prompt 模板
- ai/validator.js: JSON Schema 校验 + normalize
- ai/client.js: DeepSeek API 封装
- 金额计算唯一化，时间格式归一化

P1 - 后端架构:
- routes/: auth/rooms/messages/purchases 独立路由
- middleware/auth.js: 认证授权中间件
- ws/index.js: WebSocket 连接管理
- db.js: 启用 foreign_keys
- server.js: 从588行精简到40行入口

P2 - 前端架构:
- public/css/style.css: CSS 独立
- public/js/store.js: 全局状态 Store
- public/js/{chat,ws,purchases,auth}.js: 功能模块拆分
- index.html: 纯 HTML 结构, v2.6

server/ai/validator.js

@@ -0,0 +1,40 @@
+// JSON Schema 校验 — 验证 AI 返回的 JSON 结构
+const schemas = {
+  purchase: { required: ['purchase_item'], optional: ['quantity','unit_price','freight','payment_method','invoice_type','status','applicant','remarks','created_time','is_new','use_recent_image'] },
+  query: { required: [], optional: [] },
+  chat: { required: ['reply'], optional: [] },
+  delete: { required: ['delete_item'], optional: ['quantity','amount'] },
+  clear_all: { required: [], optional: [] },
+  ignore: { required: [], optional: [] },
+};
+
+function validate(aiResult) {
+  if (!aiResult || !aiResult.action) {
+    return { valid: false, error: '缺少 action 字段' };
+  }
+  const schema = schemas[aiResult.action];
+  if (!schema) {
+    return { valid: false, error: `未知 action: ${aiResult.action}` };
+  }
+  for (const field of schema.required) {
+    if (aiResult[field] === undefined || aiResult[field] === null) {
+      return { valid: false, error: `action=${aiResult.action} 缺少必填字段: ${field}` };
+    }
+  }
+  return { valid: true };
+}
+
+// 修复常见问题
+function normalize(aiResult) {
+  // action="ask" 兼容 reply/question
+  if (aiResult.action === 'ask') {
+    aiResult.question = aiResult.question || aiResult.reply || '请提供更多信息。';
+  }
+  // chat 兼容 reply 为空
+  if (aiResult.action === 'chat' && !aiResult.reply) {
+    aiResult.reply = '好的。';
+  }
+  return aiResult;
+}
+
+module.exports = { validate, normalize };