fix(api): 修复时光机引擎本金未折算汇率 bug，重建历史财务快照

src/actions/snapshots.ts

@@ -32,13 +32,8 @@ export async function recordDailySnapshot() {
     new Big(0)
   ).toString();
 
-  // 推导真实投入本金 CNY = 市值 - 累计盈亏
   const totalCostCny = positions.reduce(
-    (sum, pos) => {
-      const mv = new Big(pos.marketValueCny || '0');
-      const ap = new Big(pos.accumulatedPnlCny || '0');
-      return sum.plus(mv.minus(ap));
-    },
+    (sum, pos) => sum.plus(new Big(pos.totalCostCny || '0')),
     new Big(0)
   ).toString();
 
@@ -357,6 +352,7 @@ export async function reconstructPortfolioHistory() {
         quantity: transactions.quantity,
         price: transactions.price,
         fee: transactions.fee,
+        exchangeRate: transactions.exchangeRate,
       })
       .from(transactions)
       .where(lte(transactions.executedAt, currentDate))
@@ -399,7 +395,30 @@ export async function reconstructPortfolioHistory() {
       const metrics = calculateAssetMetrics(assetTxs, priceStrForMetrics);
 
       const posValueCny = new Big(metrics.marketValue).times(snapshotFxRate);
-      const posCostCny = new Big(metrics.accumulatedCost || '0');
+
+      // 使用交易时的真实汇率计算法币本金，而非直接用 metrics.accumulatedCost
+      let calculatedFiatCost = new Big(0);
+      const rawTxs = historicalTx.filter(t => t.assetId === assetId && (t.txType === 'BUY' || t.txType === 'SELL' || t.txType === 'DIVIDEND'));
+      let currentQty = new Big(0);
+      for (const tx of rawTxs) {
+        const qty = new Big(tx.quantity);
+        const fx = new Big(tx.exchangeRate || '1');
+        const price = new Big(tx.price);
+
+        if (tx.txType === 'BUY') {
+          currentQty = currentQty.plus(qty);
+          calculatedFiatCost = calculatedFiatCost.plus(qty.times(price).times(fx));
+        } else if (tx.txType === 'SELL') {
+          let avgFiatCostPerUnit = new Big(0);
+          if (currentQty.gt(0)) {
+            avgFiatCostPerUnit = calculatedFiatCost.div(currentQty);
+          }
+          calculatedFiatCost = calculatedFiatCost.minus(avgFiatCostPerUnit.times(qty));
+          currentQty = currentQty.minus(qty);
+        }
+      }
+
+      const posCostCny = calculatedFiatCost.gt(0) ? calculatedFiatCost : new Big(0);
 
       totalValueCny = totalValueCny.plus(posValueCny);
       totalCostCny = totalCostCny.plus(posCostCny);