feat: add fake trading module with crypto, stocks, forex and gold

Paper trading system with 5 commands (trade_topup, trade_buy,
trade_sell, trade_convert, trade_stats). Supports VN stocks via TCBS,
crypto via CoinGecko, forex via ER-API, and gold via PAX Gold proxy.
Per-user portfolio stored in KV with 60s price caching. 54 new tests.

plans/260414-1457-trading-module/phase-01-symbols-and-format.md

@@ -0,0 +1,102 @@
+---
+phase: 1
+title: "Symbol Registry + Formatters"
+status: Pending
+priority: P2
+effort: 45m
+---
+
+# Phase 1: Symbol Registry + Formatters
+
+## Context
+
+- [Module pattern](../../docs/adding-a-module.md)
+- [KV interface](../../src/db/kv-store-interface.js)
+
+## Overview
+
+Two pure-data/pure-function files with zero side effects. Foundation for all other phases.
+
+## File: `src/modules/trading/symbols.js`
+
+### Requirements
+
+- Export `SYMBOLS` — frozen object keyed by uppercase symbol name
+- Export `CURRENCIES` — frozen Set of supported fiat: `VND`, `USD`
+- Export helper `getSymbol(name)` — case-insensitive lookup, returns entry or `undefined`
+- Export helper `listSymbols()` — returns formatted string of all symbols grouped by category
+
+### Data shape
+
+```js
+export const SYMBOLS = Object.freeze({
+  // crypto
+  BTC:  { category: "crypto",  apiId: "bitcoin",  label: "Bitcoin" },
+  ETH:  { category: "crypto",  apiId: "ethereum", label: "Ethereum" },
+  SOL:  { category: "crypto",  apiId: "solana",   label: "Solana" },
+  // stock (Vietnam)
+  TCB:  { category: "stock",   apiId: "TCB", label: "Techcombank" },
+  VPB:  { category: "stock",   apiId: "VPB", label: "VPBank" },
+  FPT:  { category: "stock",   apiId: "FPT", label: "FPT Corp" },
+  VNM:  { category: "stock",   apiId: "VNM", label: "Vinamilk" },
+  HPG:  { category: "stock",   apiId: "HPG", label: "Hoa Phat" },
+  // others
+  GOLD: { category: "others",  apiId: "pax-gold", label: "Gold (troy oz)" },
+});
+```
+
+### Implementation steps
+
+1. Create `src/modules/trading/symbols.js`
+2. Define `SYMBOLS` constant with all entries above
+3. Define `CURRENCIES = Object.freeze(new Set(["VND", "USD"]))`
+4. `getSymbol(name)` — `SYMBOLS[name.toUpperCase()]` with guard for falsy input
+5. `listSymbols()` — group by category, format as `SYMBOL — Label` per line
+6. Keep under 60 lines
+
+---
+
+## File: `src/modules/trading/format.js`
+
+### Requirements
+
+- `formatVND(n)` — integer, dot thousands separator, suffix ` VND`. Example: `15.000.000 VND`
+- `formatUSD(n)` — 2 decimals, comma thousands, prefix `$`. Example: `$1,234.56`
+- `formatCrypto(n)` — up to 8 decimals, strip trailing zeros. Example: `0.00125`
+- `formatStock(n)` — integer (Math.floor), no decimals. Example: `150`
+- `formatAmount(n, symbol)` — dispatcher: looks up symbol category, calls correct formatter
+- `formatCurrency(n, currency)` — VND or USD formatter based on currency string
+
+### Implementation steps
+
+1. Create `src/modules/trading/format.js`
+2. `formatVND`: `Math.round(n).toLocaleString("vi-VN")` + ` VND` — verify dot separator (or manual impl for CF Workers locale support)
+3. `formatUSD`: `n.toFixed(2)` with comma grouping + `$` prefix
+4. `formatCrypto`: `parseFloat(n.toFixed(8)).toString()` to strip trailing zeros
+5. `formatStock`: `Math.floor(n).toString()`
+6. `formatAmount`: switch on `getSymbol(sym).category`
+7. `formatCurrency`: switch on currency string
+8. Keep under 80 lines
+
+### Edge cases
+
+- `formatVND(0)` -> `0 VND`
+- `formatCrypto(1.00000000)` -> `1`
+- `formatAmount` with unknown symbol -> return raw number string
+- CF Workers may not have full locale support — implement manual dot-separator for VND
+
+### Failure modes
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|-----------|--------|------------|
+| `toLocaleString` not available in CF Workers runtime | Medium | Medium | Manual formatter fallback: split on groups of 3, join with `.` |
+
+## Success criteria
+
+- [ ] `SYMBOLS` has 9 entries across 3 categories
+- [ ] `getSymbol("btc")` returns BTC entry (case-insensitive)
+- [ ] `getSymbol("NOPE")` returns `undefined`
+- [ ] `formatVND(15000000)` === `"15.000.000 VND"`
+- [ ] `formatCrypto(0.001)` === `"0.001"` (no trailing zeros)
+- [ ] `formatStock(1.7)` === `"1"`
+- [ ] Both files under 200 lines