fix(ledger): 修复价格变量泄漏与日期字符串比较陷阱，还原真实净值走势

Memory.md

@@ -160,3 +160,11 @@
 - 集成 Sonner Toast 通知：点击时显示 `toast.loading('正在重构历史走势...')`，完成后显示 `toast.success('重构成功，已填充 N 天历史数据')`，并自动刷新 `snapshots` 状态以更新 AreaChart 走势图。
 - 按钮启用 `isPending` 防重复点击，重构期间显示"重构中..."并禁用按钮。
 - 打通历史净值回溯全链路：用户从 Dashboard 首页一键触发，底层引擎自动从最早交易日起逐天计算持仓与价格，填充 `portfolio_snapshots` 表，前端图表实时渲染历史波动曲线。
+
+## 修复时光机引擎的变量泄漏与日期补零问题 (Task 51)
+- 修复了时光机循环引擎中的变量作用域泄漏导致错误复用 BTC 价格的 Bug，并标准化了 YYYY-MM-DD 日期补零逻辑以修复 SQL 字符串对比错误。
+- 在 `src/actions/snapshots.ts` 中新增 `formatDateString(date)` 辅助函数，使用 `padStart(2, '0')` 严格保证月份和日期补零，替代各处散落的 `toISOString().split('T')[0]` 调用。
+- 修复 `getHistoricalPositions` 和 `getEffectivePrice` 中的日期字符串生成逻辑：统一使用 `formatDateString(targetDate)`，确保 Drizzle 的 `lte` 查询中左右两边日期字符串格式一致（均为 `YYYY-MM-DD`），修复了 UTC 与本地时区混用导致的查询偏移。
+- 修复 `reconstructPortfolioHistory` 主循环：while 条件与 `dateStr` 生成均改用 `formatDateString(currentDate)`，确保与 `getTodayInShanghai()` 返回格式完全一致。
+- 在资产遍历循环中增加兜底防御逻辑：当 `getEffectivePrice` 返回 null 时，使用 `assetLatestPriceMap` 中缓存的 `latestPrice` 作为兜底价格，避免价格变量为 undefined 或沿用上一资产的值；同时修正了 `totalCostCny` 在 `priceStr` 为空时不应累加的 Bug。
+- 在 `allAssets` 查询中新增 `latestPrice` 字段，构建 `assetLatestPriceMap` 供兜底逻辑使用。

src/actions/snapshots.ts

@@ -6,16 +6,20 @@ import { getPortfolioPositions } from './portfolio';
 import { asc, desc, eq, gte, lte, sql } from 'drizzle-orm';
 import Big from 'big.js';
 
+function formatDateString(date: Date): string {
+  const yyyy = date.getFullYear();
+  const mm = String(date.getMonth() + 1).padStart(2, '0');
+  const dd = String(date.getDate()).padStart(2, '0');
+  return `${yyyy}-${mm}-${dd}`;
+}
+
 function getTodayInShanghai(): string {
   const now = new Date();
   const utcStr = now.toLocaleString('en-US', { timeZone: 'UTC' });
   const utcDate = new Date(utcStr);
   const shanghaiOffset = 8 * 60 * 60 * 1000;
   const shanghaiDate = new Date(utcDate.getTime() + shanghaiOffset);
-  const year = shanghaiDate.getFullYear();
-  const month = String(shanghaiDate.getMonth() + 1).padStart(2, '0');
-  const day = String(shanghaiDate.getDate()).padStart(2, '0');
-  return `${year}-${month}-${day}`;
+  return formatDateString(shanghaiDate);
 }
 
 export async function recordDailySnapshot() {
@@ -113,7 +117,7 @@ interface HistoricalPosition {
 }
 
 export async function getHistoricalPositions(targetDate: Date): Promise<HistoricalPosition[]> {
-  const dateStr = targetDate.toISOString().split('T')[0];
+  const dateStr = formatDateString(targetDate);
 
   const allTransactions = await db
     .select({
@@ -183,7 +187,7 @@ export async function getEffectivePrice(
   assetId: string,
   targetDate: Date
 ): Promise<string | null> {
-  const dateStr = targetDate.toISOString().split('T')[0];
+  const dateStr = formatDateString(targetDate);
 
   const [record] = await db
     .select({
@@ -227,11 +231,14 @@ export async function reconstructPortfolioHistory() {
     .select({
       id: assets.id,
       baseCurrency: assets.baseCurrency,
+      latestPrice: assets.latestPrice,
     })
     .from(assets);
   const assetBaseCurrencyMap = new Map<string, string>();
+  const assetLatestPriceMap = new Map<string, string>();
   for (const a of allAssets) {
     assetBaseCurrencyMap.set(a.id, a.baseCurrency);
+    assetLatestPriceMap.set(a.id, a.latestPrice || '0');
   }
 
   const allRates = await db
@@ -273,8 +280,8 @@ export async function reconstructPortfolioHistory() {
 
   let daysReconstructed = 0;
 
-  while (currentDate.toISOString().split('T')[0] <= todayStr) {
-    const dateStr = currentDate.toISOString().split('T')[0];
+  while (formatDateString(currentDate) <= todayStr) {
+    const dateStr = formatDateString(currentDate);
 
     const positions = await getHistoricalPositions(currentDate);
 
@@ -284,12 +291,21 @@ export async function reconstructPortfolioHistory() {
     for (const pos of positions) {
       const priceStr = await getEffectivePrice(pos.assetId, currentDate);
       const baseCurrency = assetBaseCurrencyMap.get(pos.assetId) || 'USD';
-      if (priceStr) {
+
+      if (!priceStr) {
+        const fallbackPrice = assetLatestPriceMap.get(pos.assetId) || '0';
+        const cnyPrice = convertPriceToCny(fallbackPrice, baseCurrency);
+        const price = new Big(cnyPrice);
+        const qty = new Big(pos.quantity);
+        totalValueCny = totalValueCny.plus(price.times(qty));
+        totalCostCny = totalCostCny.plus(pos.totalCost);
+        continue;
+      }
+
       const cnyPrice = convertPriceToCny(priceStr, baseCurrency);
       const price = new Big(cnyPrice);
       const qty = new Big(pos.quantity);
       totalValueCny = totalValueCny.plus(price.times(qty));
-      }
       totalCostCny = totalCostCny.plus(pos.totalCost);
     }