Add demo account service

app/demo_account.py

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+from datetime import datetime, timezone
+from pydantic import BaseModel, Field
+
+
+class DemoTrade(BaseModel):
+    id: int
+    market: str
+    side: str
+    price: float
+    amount: float
+    quote_amount: float
+    reason: str
+    created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+
+
+class DemoAccount(BaseModel):
+    quote_asset: str
+    balance: float
+    equity: float
+    realized_pnl: float = 0.0
+    trades: list[DemoTrade] = Field(default_factory=list)
+
+
+class DemoAccountService:
+    def __init__(self, initial_balance: float, quote_asset: str) -> None:
+        self.initial_balance = initial_balance
+        self.quote_asset = quote_asset
+        self.account = DemoAccount(
+            quote_asset=quote_asset,
+            balance=initial_balance,
+            equity=initial_balance,
+        )
+
+    def snapshot(self) -> DemoAccount:
+        return self.account
+
+    def reset(self) -> DemoAccount:
+        self.account = DemoAccount(
+            quote_asset=self.quote_asset,
+            balance=self.initial_balance,
+            equity=self.initial_balance,
+        )
+        return self.account
+
+    def add_demo_trade(self, market: str, side: str, price: float, amount: float, reason: str) -> DemoTrade:
+        if price <= 0 or amount <= 0:
+            raise ValueError('price and amount must be positive')
+
+        quote_amount = price * amount
+        side_normalized = side.lower()
+        if side_normalized not in {'buy', 'sell'}:
+            raise ValueError('side must be buy or sell')
+
+        if side_normalized == 'buy':
+            if quote_amount > self.account.balance:
+                raise ValueError('not enough demo balance')
+            self.account.balance -= quote_amount
+        else:
+            self.account.balance += quote_amount
+            self.account.realized_pnl += quote_amount
+
+        self.account.equity = self.account.balance
+        trade = DemoTrade(
+            id=len(self.account.trades) + 1,
+            market=market.upper(),
+            side=side_normalized,
+            price=price,
+            amount=amount,
+            quote_amount=quote_amount,
+            reason=reason,
+        )
+        self.account.trades.append(trade)
+        return trade