feat: use real BIDV bid/ask rates for forex conversion

Replace hardcoded 0.5% spread with live buy/sell rates from BIDV bank
API. Buying USD uses bank's sell rate (higher), selling USD uses bank's
buy rate (lower). Reply shows both rates and actual spread percentage.

src/modules/trading/README.md

@@ -9,7 +9,7 @@ Paper-trading system where each Telegram user manages a virtual portfolio.
 | `/trade_topup <amount>` | Add VND to account. Tracks cumulative invested via `totalvnd`. |
 | `/trade_buy <amount> <symbol>` | Buy at market price, deducting VND. Stocks must be integer quantities. |
 | `/trade_sell <amount> <symbol>` | Sell holdings back to VND at market price. |
-| `/trade_convert <amount> <from> <to>` | Convert between currencies with bid/ask spread (0.5%). |
+| `/trade_convert <amount> <from> <to>` | Convert between currencies at real BIDV bid/ask rates. |
 | `/trade_stats` | Portfolio breakdown with all assets valued in VND, plus P&L vs invested. |
 
 ## Supported Symbols
@@ -62,7 +62,7 @@ KV namespace prefix: `trading:`
   "ts": 1713100000000,
   "crypto": { "BTC": 1500000000, "ETH": 50000000, "SOL": 3000000 },
   "stock": { "TCB": 25000, "VPB": 18000, "FPT": 120000, "VNM": 70000, "HPG": 28000 },
-  "forex": { "USD": 25400 },
+  "forex": { "USD": { "mid": 25400, "buy": 25200, "sell": 25600 } },
   "others": { "GOLD": 72000000 }
 }
 ```
@@ -79,7 +79,7 @@ Three free APIs fetched in parallel, cached in KV for 60 seconds:
 |-----|---------|------|-----------|
 | CoinGecko `/api/v3/simple/price` | Crypto + gold prices in VND | None | 30 calls/min (free) |
 | TCBS `/stock-insight/v1/stock/bars-long-term` | Vietnam stock close prices (× 1000) | None | Unofficial |
-| open.er-api.com `/v6/latest/USD` | USD/VND forex rate | None | 1,500/month (free) |
+| BIDV `/ServicesBIDV/ExchangeDetailServlet` | USD/VND buy/sell rates | None | Unofficial |
 
 On partial API failure, available data is returned. On total failure, stale cache up to 5 minutes old is used before surfacing an error.
 

src/modules/trading/handlers.js

@@ -12,12 +12,9 @@ import {
   getPortfolio,
   savePortfolio,
 } from "./portfolio.js";
-import { getForexRate, getPrice } from "./prices.js";
+import { getForexBidAsk, getPrice } from "./prices.js";
 import { CURRENCIES, getSymbol, listSymbols } from "./symbols.js";
 
-/** Bid/ask spread for forex conversion (0.5% each side of mid rate) */
-const FOREX_SPREAD = 0.005;
-
 function uid(ctx) {
   return ctx.from?.id;
 }
@@ -135,38 +132,36 @@ export async function handleConvert(ctx, db) {
     return ctx.reply(`Supported currencies: ${[...CURRENCIES].join(", ")}`);
   if (from === to) return ctx.reply("Cannot convert to the same currency.");
 
-  let midRate;
+  let rates;
   try {
-    midRate = await getForexRate(db, "USD");
+    rates = await getForexBidAsk(db, "USD");
   } catch {
     return ctx.reply("Could not fetch forex rate. Try again later.");
   }
-  if (midRate == null) return ctx.reply("Forex rate unavailable. Try again later.");
-
-  // Buying foreign currency costs more (ask), selling it back gives less (bid)
-  const buyRate = midRate * (1 + FOREX_SPREAD); // VND per USD when buying USD
-  const sellRate = midRate * (1 - FOREX_SPREAD); // VND per USD when selling USD
+  if (!rates) return ctx.reply("Forex rate unavailable. Try again later.");
 
   const p = await getPortfolio(db, uid(ctx));
   const result = deductCurrency(p, from, amount);
   if (!result.ok)
     return ctx.reply(`Insufficient ${from}. Balance: ${formatCurrency(result.balance, from)}`);
 
+  // buy = bank buys USD (you sell USD → VND), sell = bank sells USD (you buy USD → pay VND)
   let converted;
   let rateUsed;
   if (from === "VND" && to === "USD") {
-    // buying USD with VND — pay ask price
-    converted = amount / buyRate;
-    rateUsed = buyRate;
+    // you're buying USD from bank → bank sells at higher price
+    converted = amount / rates.sell;
+    rateUsed = rates.sell;
   } else {
-    // selling USD for VND — receive bid price
-    converted = amount * sellRate;
-    rateUsed = sellRate;
+    // you're selling USD to bank → bank buys at lower price
+    converted = amount * rates.buy;
+    rateUsed = rates.buy;
   }
 
   addCurrency(p, to, converted);
   await savePortfolio(db, uid(ctx), p);
+  const spread = (((rates.sell - rates.buy) / rates.buy) * 100).toFixed(2);
   await ctx.reply(
-    `Converted ${formatCurrency(amount, from)} → ${formatCurrency(converted, to)}\nRate: ${formatVND(rateUsed)}/USD (spread: ${(FOREX_SPREAD * 100).toFixed(1)}%)`,
+    `Converted ${formatCurrency(amount, from)} → ${formatCurrency(converted, to)}\nRate: ${formatVND(rateUsed)}/USD (buy: ${formatVND(rates.buy)}, sell: ${formatVND(rates.sell)}, spread: ${spread}%)`,
   );
 }

src/modules/trading/prices.js

@@ -54,14 +54,21 @@ async function fetchStocks() {
   return stock;
 }
 
-/** Forex rates via open.er-api.com — VND per 1 USD */
+/** Forex rates via BIDV public API — returns real buy/sell rates */
 async function fetchForex() {
-  const res = await fetch("https://open.er-api.com/v6/latest/USD");
-  if (!res.ok) throw new Error(`Forex API ${res.status}`);
-  const data = await res.json();
-  const vndRate = data?.rates?.VND;
-  if (vndRate == null) throw new Error("Forex missing VND rate");
-  return { USD: vndRate };
+  const res = await fetch("https://www.bidv.com.vn/ServicesBIDV/ExchangeDetailServlet");
+  if (!res.ok) throw new Error(`BIDV API ${res.status}`);
+  const json = await res.json();
+  const usd = json?.data?.find((r) => r.currency === "USD");
+  if (!usd) throw new Error("BIDV missing USD rate");
+  // muaCk = bank buy (transfer), ban = bank sell — parse "26,141" → 26141
+  const parse = (s) => Number.parseFloat(String(s).replace(/,/g, ""));
+  const buy = parse(usd.muaCk);
+  const sell = parse(usd.ban);
+  if (!Number.isFinite(buy) || !Number.isFinite(sell)) {
+    throw new Error("BIDV invalid USD rate");
+  }
+  return { USD: { mid: (buy + sell) / 2, buy, sell } };
 }
 
 /**
@@ -119,13 +126,28 @@ export async function getPrice(db, symbol) {
 }
 
 /**
- * VND equivalent of 1 unit of currency.
+ * Mid-rate VND equivalent of 1 unit of currency (for stats display).
  * @param {import("../../db/kv-store-interface.js").KVStore} db
  * @param {string} currency — "VND" or "USD"
- * @returns {Promise<number>}
+ * @returns {Promise<number|null>}
  */
 export async function getForexRate(db, currency) {
   if (currency === "VND") return 1;
   const prices = await getPrices(db);
-  return prices.forex?.[currency] ?? null;
+  return prices.forex?.[currency]?.mid ?? null;
+}
+
+/**
+ * Buy/sell forex rates for a currency.
+ * buy = bank buys USD from you (you sell USD → get VND), sell = bank sells USD to you (you buy USD → pay VND).
+ * @param {import("../../db/kv-store-interface.js").KVStore} db
+ * @param {string} currency
+ * @returns {Promise<{ buy: number, sell: number }|null>}
+ */
+export async function getForexBidAsk(db, currency) {
+  if (currency === "VND") return null;
+  const prices = await getPrices(db);
+  const rate = prices.forex?.[currency];
+  if (!rate?.buy || !rate?.sell) return null;
+  return { buy: rate.buy, sell: rate.sell };
 }

src/modules/trading/stats-handler.js

@@ -23,7 +23,7 @@ export async function handleStats(ctx, db) {
   const currLines = [];
   for (const [cur, bal] of Object.entries(p.currency)) {
     if (bal === 0) continue;
-    const rate = cur === "VND" ? 1 : (prices.forex?.[cur] ?? 0);
+    const rate = cur === "VND" ? 1 : (prices.forex?.[cur]?.mid ?? 0);
     const vndVal = bal * rate;
     totalValue += vndVal;
     currLines.push(