Add trailing stops and parameter sweeps

Trailing stops: strategies can pass trailing_stop_pct to enter_long/enter_short.
advance_trailing_stop in app/engine/portfolio.py ratchets the stop with the price
(up for longs, down for shorts) and never loosens; the engine persists the
ratcheted level and the backtester applies the same logic. Position gains a
trailing_stop_pct column (additive migration). momentum exposes it as a param.

Parameter sweeps: app/backtest/sweep.py runs a cartesian grid through the same
backtester and ranks by return minus max drawdown. POST /api/sweep plus a
dashboard Parameter sweep panel.

backend/tests/test_portfolio.py

@@ -4,6 +4,7 @@ import pytest
 
 from app.engine.portfolio import (
     account_equity,
+    advance_trailing_stop,
     apply_close,
     apply_open,
     order_qty,
@@ -57,3 +58,30 @@ def test_risk_exit_short() -> None:
     assert risk_exit(PositionSide.SHORT, 110, 90, high=111, low=100) == ("stop loss", 110)
     assert risk_exit(PositionSide.SHORT, 110, 90, high=100, low=89) == ("take profit", 90)
     assert risk_exit(PositionSide.SHORT, None, None, high=100, low=95) is None
+
+
+def test_trailing_stop_long_ratchets_up_only() -> None:
+    # First update seeds the stop 2% below the high.
+    stop = advance_trailing_stop(PositionSide.LONG, None, 0.02, high=100, low=95)
+    assert stop == pytest.approx(98.0)
+    # A higher high raises it.
+    stop = advance_trailing_stop(PositionSide.LONG, stop, 0.02, high=110, low=100)
+    assert stop == pytest.approx(107.8)
+    # A pullback never loosens it.
+    stop = advance_trailing_stop(PositionSide.LONG, stop, 0.02, high=105, low=100)
+    assert stop == pytest.approx(107.8)
+
+
+def test_trailing_stop_short_ratchets_down_only() -> None:
+    stop = advance_trailing_stop(PositionSide.SHORT, None, 0.02, high=100, low=95)
+    assert stop == pytest.approx(96.9)
+    stop = advance_trailing_stop(PositionSide.SHORT, stop, 0.02, high=95, low=90)
+    assert stop == pytest.approx(91.8)
+    # A bounce never loosens the short's stop.
+    stop = advance_trailing_stop(PositionSide.SHORT, stop, 0.02, high=95, low=92)
+    assert stop == pytest.approx(91.8)
+
+
+def test_trailing_stop_disabled_is_noop() -> None:
+    assert advance_trailing_stop(PositionSide.LONG, 90.0, 0.0, high=100, low=80) == 90.0
+    assert advance_trailing_stop(PositionSide.LONG, None, 0.0, high=100, low=80) is None