refactor(ledger): PnL 引擎接入动态汇率字典，实现跨币种资产的高精度法币折算

Memory.md

@@ -279,3 +279,9 @@
 - 核心修复：将 `avgCost: avgCost.toString()` 替换为 `avgCost: metrics.averageCost`，将 `dilutedCost: dilutedCost.toString()` 替换为 `dilutedCost: metrics.dilutedCost`。
 - 同时新增 `floatingPnl` 和 `accumulatedPnl` 字段映射到 Position 接口，补齐了财务引擎产出的六大核心指标中缺失的两个字段。
 - 遵循 `metrics` 返回值已是 string 类型的规范，不再调用 `.toString()` 导致冗余转换。
+
+## 重构 portfolio API，废弃静态 asset.exchangeRate，全面接入 exchange_rates_history 动态汇率流水表，通过 O(1) 内存字典提升跨币种折算精度与性能 (Task 63b)
+- 在 `src/actions/portfolio.ts` 顶部新增 `getLatestRatesMap()` 辅助函数：通过 Drizzle ORM 的 `orderBy(desc(fetchTime)).limit(1)` 分别查询 `exchangeRatesHistory` 表中 `USD→CNY` 与 `HKD→CNY` 的最新一条记录，组装为 `Record<string, Big>` 字典（`{ CNY: 1, USD: dbUsd?.rate || 7.2, HKD: dbHkd?.rate || 0.9 }`），内置查不到时的兜底安全值。
+- 废弃 `getPortfolioPositions` 中对静态 `exchangeRates` 表的 N+1 查询：在函数顶部调用 `getLatestRatesMap()` 获取动态汇率字典，并将其转换为 `Map<string, string>` 供 `calculateCnyValueFromPrice` 等下游函数继续使用。
+- 替换 PnL 映射逻辑中的静态汇率查找：将 `getRate(rateMap, holding.baseCurrency, 'CNY')` 改为直接从 `dynamicRateMap[holding.baseCurrency]` 取值，实现 O(1) 内存字典访问，消除数据库耦合。
+- 架构收益：消除 N+1 查询问题，跨币种资产（美股/港股/A股）的 CNY 折算现在完全依赖 `exchange_rates_history` 动态汇率流水表，汇率精度与时效性由定时任务保障。

src/actions/portfolio.ts

@@ -1,9 +1,9 @@
 'use server';
 
 import { db } from '@/db';
-import { transactions, assets, exchangeRates } from '@/db/schema';
+import { transactions, assets, exchangeRates, exchangeRatesHistory } from '@/db/schema';
 import Big from 'big.js';
-import { asc, eq } from 'drizzle-orm';
+import { asc, desc, eq } from 'drizzle-orm';
 import { calculateAssetMetrics } from '@/utils/finance';
 
 interface Position {
@@ -64,6 +64,35 @@ interface RawRate {
   rate: string;
 }
 
+async function getLatestRatesMap(): Promise<Record<string, Big>> {
+  const usdResult = await db
+    .select({
+      rate: exchangeRatesHistory.rate,
+    })
+    .from(exchangeRatesHistory)
+    .where(eq(exchangeRatesHistory.fromCurrency, 'USD'))
+    .orderBy(desc(exchangeRatesHistory.fetchTime))
+    .limit(1);
+
+  const hkdResult = await db
+    .select({
+      rate: exchangeRatesHistory.rate,
+    })
+    .from(exchangeRatesHistory)
+    .where(eq(exchangeRatesHistory.fromCurrency, 'HKD'))
+    .orderBy(desc(exchangeRatesHistory.fetchTime))
+    .limit(1);
+
+  const dbUsd = usdResult[0];
+  const dbHkd = hkdResult[0];
+
+  return {
+    CNY: new Big(1),
+    USD: new Big(dbUsd?.rate || 7.2),
+    HKD: new Big(dbHkd?.rate || 0.9),
+  };
+}
+
 function buildRateMap(rates: RawRate[]): Map<string, string> {
   const map = new Map<string, string>();
   for (const r of rates) {
@@ -167,13 +196,14 @@ export async function getPortfolioPositions(): Promise<Position[]> {
     .leftJoin(assets, eq(assets.id, transactions.assetId))
     .orderBy(asc(transactions.executedAt));
 
-  const rates = await db.select({
-    fromCurrency: exchangeRates.fromCurrency,
-    toCurrency: exchangeRates.toCurrency,
-    rate: exchangeRates.rate,
-  }).from(exchangeRates);
+  const dynamicRateMap = await getLatestRatesMap();
 
-  const rateMap = buildRateMap(rates);
+  const rateMap = new Map<string, string>();
+  for (const [currency, rate] of Object.entries(dynamicRateMap)) {
+    if (currency !== 'CNY') {
+      rateMap.set(`${currency}_CNY`, rate.toString());
+    }
+  }
 
   const holdings = new Map<string, {
     assetId: string;
@@ -330,10 +360,9 @@ export async function getPortfolioPositions(): Promise<Position[]> {
       holding.latestPrice
     );
 
-    // 获取资产对人民币的汇率
-    const fxRate = new Big(
-      getRate(rateMap, holding.baseCurrency, 'CNY') || '1'
-    );
+    // 从动态汇率字典获取资产对人民币的汇率
+    const currencyKey = holding.baseCurrency || 'CNY';
+    const fxRate = dynamicRateMap[currencyKey] || new Big(1);
 
     // 将引擎返回的原生币种金额折算为 CNY
     const marketValueCny = new Big(metrics.marketValue).times(fxRate).toString();