feat(backtester): Phase 1 complete — realistic backtesting engine

- Slippage modeling (configurable per-trade, buy higher/sell lower)
- Trading fee deduction (maker/taker configurable)
- Stop-loss and take-profit auto-execution per position
- Short selling support (allow_short flag)
- Walk-forward analysis engine (in-sample/out-of-sample, efficiency ratio)
- Daily equity curve Sharpe/Sortino with risk-free rate adjustment
- Recovery factor, consecutive win/loss streaks, fee-aware PnL
- 312 tests passing

6 files changed, 86 insertions, 40 deletions

diff --git a/services/backtester/src/backtester/metrics.py b/services/backtester/src/backtester/metrics.py
index 5b43afd..239cb6f 100644
--- a/services/backtester/src/backtester/metrics.py
+++ b/services/backtester/src/backtester/metrics.py
@@ -207,7 +207,7 @@ def compute_detailed_metrics(
 
         # Sortino (downside deviation of excess returns)
         downside = [min(r - daily_rf, 0.0) for r in daily_returns]
-        downside_var = sum(d ** 2 for d in downside) / len(downside)
+        downside_var = sum(d**2 for d in downside) / len(downside)
         downside_std = math.sqrt(downside_var)
         sortino = (mean_excess / downside_std * math.sqrt(365)) if downside_std > 0 else 0.0
     else:
diff --git a/services/backtester/src/backtester/simulator.py b/services/backtester/src/backtester/simulator.py
index 1e37740..64c88dd 100644
--- a/services/backtester/src/backtester/simulator.py
+++ b/services/backtester/src/backtester/simulator.py
@@ -89,9 +89,7 @@ class OrderSimulator:
 
             if triggered and exit_price is not None:
                 # Close the position
-                close_side = (
-                    OrderSide.SELL if pos.side == OrderSide.BUY else OrderSide.BUY
-                )
+                close_side = OrderSide.SELL if pos.side == OrderSide.BUY else OrderSide.BUY
                 fee = self._calculate_fee(exit_price, pos.quantity)
 
                 if pos.side == OrderSide.BUY:
@@ -198,9 +196,7 @@ class OrderSimulator:
         )
         return True
 
-    def _close_open_position(
-        self, symbol: str, side: OrderSide, quantity: Decimal
-    ) -> None:
+    def _close_open_position(self, symbol: str, side: OrderSide, quantity: Decimal) -> None:
         """Remove closed quantity from open positions (FIFO)."""
         remaining_qty = quantity
         new_positions: list[OpenPosition] = []
diff --git a/services/backtester/src/backtester/walk_forward.py b/services/backtester/src/backtester/walk_forward.py
index fe6d020..c7b7fd8 100644
--- a/services/backtester/src/backtester/walk_forward.py
+++ b/services/backtester/src/backtester/walk_forward.py
@@ -1,4 +1,5 @@
 """Walk-forward analysis for strategy parameter optimization."""
+
 from dataclasses import dataclass, field
 from decimal import Decimal
 from typing import Callable
@@ -10,6 +11,7 @@ from backtester.engine import BacktestEngine, BacktestResult, StrategyProtocol
 @dataclass
 class WalkForwardWindow:
     """Result for a single in-sample/out-of-sample window."""
+
     window_index: int
     in_sample_result: BacktestResult
     out_of_sample_result: BacktestResult
@@ -19,6 +21,7 @@ class WalkForwardWindow:
 @dataclass
 class WalkForwardResult:
     """Aggregated walk-forward analysis results."""
+
     strategy_name: str
     symbol: str
     num_windows: int
@@ -130,12 +133,14 @@ class WalkForwardEngine:
             engine = BacktestEngine(strategy, self._initial_balance)
             oos_result = engine.run(out_of_sample)
 
-            windows.append(WalkForwardWindow(
-                window_index=i,
-                in_sample_result=best_is_result,
-                out_of_sample_result=oos_result,
-                best_params=best_params,
-            ))
+            windows.append(
+                WalkForwardWindow(
+                    window_index=i,
+                    in_sample_result=best_is_result,
+                    out_of_sample_result=oos_result,
+                    best_params=best_params,
+                )
+            )
 
         return WalkForwardResult(
             strategy_name=strategy_name,
diff --git a/services/backtester/tests/test_engine.py b/services/backtester/tests/test_engine.py
index 003e951..743a43b 100644
--- a/services/backtester/tests/test_engine.py
+++ b/services/backtester/tests/test_engine.py
@@ -44,9 +44,7 @@ def test_backtest_engine_runs_strategy_over_candles():
     strategy.on_candle.return_value = None
 
     candles = make_candles([50000.0, 51000.0, 52000.0])
-    engine = BacktestEngine(
-        strategy, Decimal("10000"), slippage_pct=0.0, taker_fee_pct=0.0
-    )
+    engine = BacktestEngine(strategy, Decimal("10000"), slippage_pct=0.0, taker_fee_pct=0.0)
     result = engine.run(candles)
 
     assert strategy.on_candle.call_count == 3
@@ -64,9 +62,7 @@ def test_backtest_engine_executes_signals():
     strategy.on_candle.side_effect = [buy_signal, None, sell_signal]
 
     candles = make_candles([50000.0, 52000.0, 55000.0])
-    engine = BacktestEngine(
-        strategy, Decimal("10000"), slippage_pct=0.0, taker_fee_pct=0.0
-    )
+    engine = BacktestEngine(strategy, Decimal("10000"), slippage_pct=0.0, taker_fee_pct=0.0)
     result = engine.run(candles)
 
     assert result.total_trades == 2
diff --git a/services/backtester/tests/test_metrics.py b/services/backtester/tests/test_metrics.py
index 34314b3..582309a 100644
--- a/services/backtester/tests/test_metrics.py
+++ b/services/backtester/tests/test_metrics.py
@@ -111,11 +111,11 @@ def test_consecutive_losses():
     """Consecutive loss tracking should count streaks correctly."""
     trades = [
         _make_trade("BUY", "100", 0),
-        _make_trade("SELL", "110", 10),   # win
+        _make_trade("SELL", "110", 10),  # win
         _make_trade("BUY", "110", 20),
-        _make_trade("SELL", "105", 30),   # loss
+        _make_trade("SELL", "105", 30),  # loss
         _make_trade("BUY", "105", 40),
-        _make_trade("SELL", "100", 50),   # loss
+        _make_trade("SELL", "100", 50),  # loss
     ]
     metrics = compute_detailed_metrics(trades, Decimal("10000"), Decimal("10005"))
     assert metrics.max_consecutive_losses >= 1
@@ -126,12 +126,44 @@ def test_risk_free_rate_affects_sharpe():
     """Higher risk-free rate should lower Sharpe ratio."""
     base = datetime(2025, 1, 1, tzinfo=timezone.utc)
     trades = [
-        TradeRecord(time=base, symbol="BTCUSDT", side="BUY", price=Decimal("100"), quantity=Decimal("1")),
-        TradeRecord(time=base + timedelta(days=1), symbol="BTCUSDT", side="SELL", price=Decimal("110"), quantity=Decimal("1")),
-        TradeRecord(time=base + timedelta(days=2), symbol="BTCUSDT", side="BUY", price=Decimal("105"), quantity=Decimal("1")),
-        TradeRecord(time=base + timedelta(days=3), symbol="BTCUSDT", side="SELL", price=Decimal("115"), quantity=Decimal("1")),
-        TradeRecord(time=base + timedelta(days=4), symbol="BTCUSDT", side="BUY", price=Decimal("110"), quantity=Decimal("1")),
-        TradeRecord(time=base + timedelta(days=5), symbol="BTCUSDT", side="SELL", price=Decimal("108"), quantity=Decimal("1")),
+        TradeRecord(
+            time=base, symbol="BTCUSDT", side="BUY", price=Decimal("100"), quantity=Decimal("1")
+        ),
+        TradeRecord(
+            time=base + timedelta(days=1),
+            symbol="BTCUSDT",
+            side="SELL",
+            price=Decimal("110"),
+            quantity=Decimal("1"),
+        ),
+        TradeRecord(
+            time=base + timedelta(days=2),
+            symbol="BTCUSDT",
+            side="BUY",
+            price=Decimal("105"),
+            quantity=Decimal("1"),
+        ),
+        TradeRecord(
+            time=base + timedelta(days=3),
+            symbol="BTCUSDT",
+            side="SELL",
+            price=Decimal("115"),
+            quantity=Decimal("1"),
+        ),
+        TradeRecord(
+            time=base + timedelta(days=4),
+            symbol="BTCUSDT",
+            side="BUY",
+            price=Decimal("110"),
+            quantity=Decimal("1"),
+        ),
+        TradeRecord(
+            time=base + timedelta(days=5),
+            symbol="BTCUSDT",
+            side="SELL",
+            price=Decimal("108"),
+            quantity=Decimal("1"),
+        ),
     ]
     m1 = compute_detailed_metrics(trades, Decimal("10000"), Decimal("10018"), risk_free_rate=0.0)
     m2 = compute_detailed_metrics(trades, Decimal("10000"), Decimal("10018"), risk_free_rate=0.10)
@@ -154,8 +186,22 @@ def test_fee_subtracted_from_pnl():
     """Fees should be subtracted from trade PnL."""
     base = datetime(2025, 1, 1, tzinfo=timezone.utc)
     trades_with_fees = [
-        TradeRecord(time=base, symbol="BTC", side="BUY", price=Decimal("100"), quantity=Decimal("1"), fee=Decimal("1")),
-        TradeRecord(time=base + timedelta(minutes=10), symbol="BTC", side="SELL", price=Decimal("110"), quantity=Decimal("1"), fee=Decimal("1")),
+        TradeRecord(
+            time=base,
+            symbol="BTC",
+            side="BUY",
+            price=Decimal("100"),
+            quantity=Decimal("1"),
+            fee=Decimal("1"),
+        ),
+        TradeRecord(
+            time=base + timedelta(minutes=10),
+            symbol="BTC",
+            side="SELL",
+            price=Decimal("110"),
+            quantity=Decimal("1"),
+            fee=Decimal("1"),
+        ),
     ]
     # PnL should be 10 - 1 - 1 = 8
     metrics = compute_detailed_metrics(trades_with_fees, Decimal("10000"), Decimal("10008"))
diff --git a/services/backtester/tests/test_walk_forward.py b/services/backtester/tests/test_walk_forward.py
index e672dac..5ab2e7b 100644
--- a/services/backtester/tests/test_walk_forward.py
+++ b/services/backtester/tests/test_walk_forward.py
@@ -1,10 +1,10 @@
 """Tests for walk-forward analysis."""
+
 import sys
 from pathlib import Path
 from decimal import Decimal
 from datetime import datetime, timedelta, timezone
 
-import pytest
 
 sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
 sys.path.insert(0, str(Path(__file__).resolve().parents[2] / "strategy-engine"))
@@ -19,15 +19,18 @@ def _generate_candles(n=100, base_price=100.0):
     for i in range(n):
         # Simple oscillating price
         price = base_price + (i % 20) - 10
-        candles.append(Candle(
-            symbol="BTCUSDT", timeframe="1h",
-            open_time=datetime(2025, 1, 1, tzinfo=timezone.utc) + timedelta(hours=i),
-            open=Decimal(str(price)),
-            high=Decimal(str(price + 5)),
-            low=Decimal(str(price - 5)),
-            close=Decimal(str(price)),
-            volume=Decimal("100"),
-        ))
+        candles.append(
+            Candle(
+                symbol="BTCUSDT",
+                timeframe="1h",
+                open_time=datetime(2025, 1, 1, tzinfo=timezone.utc) + timedelta(hours=i),
+                open=Decimal(str(price)),
+                high=Decimal(str(price + 5)),
+                low=Decimal(str(price - 5)),
+                close=Decimal(str(price)),
+                volume=Decimal("100"),
+            )
+        )
     return candles