Add the Williams %R strategy

New williams_r indicator (in [-100, 0]) exposed as ctx.williams_r(period), and a
long-only mean-reversion strategy: enter when %R is at/below the oversold level,
exit when it reaches the overbought level, with an optional ATR stop. 11
strategies total. Adds indicator and strategy tests; smoke-tested on live
BTC/USDT 5m (backtest + parameter sweep).

backend/tests/test_williams_r.py

@@ -0,0 +1,87 @@
+from __future__ import annotations
+
+import pytest
+
+from app.engine.indicators import williams_r
+from app.engine.strategies import get_strategy
+from app.engine.strategies.base import StrategyContext
+from app.models import Position, PositionSide
+from app.schemas import Candle
+
+
+def candles(closes: list[float]) -> list[Candle]:
+    return [
+        Candle(ts=i, open=c, high=c * 1.001, low=c * 0.999, close=c, volume=1.0)
+        for i, c in enumerate(closes)
+    ]
+
+
+def ctx(closes: list[float], position=None) -> StrategyContext:
+    return StrategyContext(candles=candles(closes), position=position, equity=1000.0, cash=1000.0)
+
+
+# --- indicator ------------------------------------------------------------
+def test_williams_r_bounds() -> None:
+    series = williams_r(candles([100, 102, 98, 101, 99, 103, 97, 100]), 5)
+    for value in series:
+        if value is not None:
+            assert -100.0 <= value <= 0.0
+
+
+def test_williams_r_rising_is_overbought() -> None:
+    rising = [float(i) for i in range(1, 30)]
+    value = williams_r(candles(rising), 14)[-1]
+    assert value is not None and value > -50  # near the top of the range
+
+
+def test_williams_r_falling_is_oversold() -> None:
+    falling = [float(i) for i in range(30, 1, -1)]
+    value = williams_r(candles(falling), 14)[-1]
+    assert value is not None and value < -50  # near the bottom of the range
+
+
+def test_williams_r_flat_series_is_neutral() -> None:
+    flat = [100.0] * 20
+    assert williams_r(candles(flat), 14)[-1] == pytest.approx(-50.0)
+
+
+# --- strategy -------------------------------------------------------------
+def test_williams_r_registered() -> None:
+    from app.engine.strategies import list_strategies
+
+    assert "williams_r" in {c.key for c in list_strategies()}
+
+
+def test_enters_long_when_oversold() -> None:
+    strategy = get_strategy("williams_r")({"period": 5, "oversold": -80, "atr_stop_mult": 0})
+    falling = [100 - i for i in range(15)]
+    c = ctx([float(x) for x in falling])
+    strategy.on_candle(c)
+    intents = c.take_intents()
+    assert intents and intents[0].action == "enter_long"
+
+
+def test_exits_when_overbought() -> None:
+    strategy = get_strategy("williams_r")({"period": 5, "overbought": -20, "atr_stop_mult": 0})
+    position = Position(
+        strategy_key="williams_r",
+        symbol="BTC/USDT",
+        side=PositionSide.LONG,
+        qty=1.0,
+        entry_price=100.0,
+    )
+    rising = [100 + i for i in range(15)]
+    c = ctx([float(x) for x in rising], position=position)
+    strategy.on_candle(c)
+    intents = c.take_intents()
+    assert intents and intents[0].action == "close"
+
+
+def test_atr_stop_is_set_when_enabled() -> None:
+    strategy = get_strategy("williams_r")({"period": 5, "oversold": -80, "atr_stop_mult": 2.0})
+    falling = [float(100 - i) for i in range(15)]
+    c = ctx(falling)
+    strategy.on_candle(c)
+    intents = c.take_intents()
+    assert intents and intents[0].action == "enter_long"
+    assert intents[0].stop_loss is not None