Add explicit trade sizing

src/llm_trade_bot/risk.py

@@ -23,6 +23,8 @@ class OrderValidator:
         request: OpenTradeRequest,
         account: AccountSnapshot,
         instrument: InstrumentRules,
+        quote_notional: Decimal | None = None,
+        available_balance_percent: Decimal | None = None,
     ) -> ValidatedOrder:
         if account.equity <= 0:
             raise OrderRejected("Account equity must be positive")
@@ -38,8 +40,27 @@ class OrderValidator:
         if leverage < 1 or leverage > instrument.max_leverage:
             raise OrderRejected(f"Leverage must be between 1 and {instrument.max_leverage}")
 
-        buying_power = account.available_balance * leverage
-        quantity = buying_power / entry
+        if quote_notional is not None and available_balance_percent is not None:
+            raise OrderRejected("Set either quote_notional or available_balance_percent, not both")
+        if quote_notional is not None and quote_notional <= 0:
+            raise OrderRejected("quote_notional must be positive")
+        if available_balance_percent is not None and not 0 < available_balance_percent <= 100:
+            raise OrderRejected("available_balance_percent must be in (0, 100]")
+
+        maximum_notional = account.available_balance * leverage
+        if quote_notional is not None:
+            requested_notional = quote_notional
+        elif available_balance_percent is not None:
+            requested_notional = maximum_notional * available_balance_percent / Decimal(100)
+        else:
+            requested_notional = maximum_notional
+        if requested_notional > maximum_notional:
+            raise OrderRejected(
+                f"Requested quote notional {requested_notional} exceeds available buying power "
+                f"{maximum_notional} (available_balance={account.available_balance}, leverage={leverage})"
+            )
+
+        quantity = requested_notional / entry
         quantity = _round_down(quantity, instrument.qty_step)
         notional = quantity * entry
         if quantity < instrument.min_qty: