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/app/engine/engine.py

@@ -10,6 +10,7 @@ from app.broker.base import Broker
 from app.config import Settings
 from app.engine.portfolio import (
     account_equity,
+    advance_trailing_stop,
     apply_close,
     apply_open,
     order_qty,
@@ -202,6 +203,7 @@ class TradingEngine:
                     size=intent.size if intent.size is not None else 1.0,
                     stop_loss=intent.stop_loss,
                     take_profit=intent.take_profit,
+                    trailing_stop_pct=intent.trailing_stop_pct,
                     reason=intent.reason or intent.action,
                 )
 
@@ -213,6 +215,7 @@ class TradingEngine:
         stop_loss: float | None,
         take_profit: float | None,
         reason: str,
+        trailing_stop_pct: float = 0.0,
         qty_override: float | None = None,
     ) -> None:
         price = self.last_price
@@ -250,6 +253,7 @@ class TradingEngine:
             entry_fee=fill.fee,
             stop_loss=stop_loss,
             take_profit=take_profit,
+            trailing_stop_pct=trailing_stop_pct,
         )
         self.store.record_order(
             position_id=self.position.id,
@@ -317,6 +321,21 @@ class TradingEngine:
         position = self.position
         if position is None:
             return
+
+        # Ratchet a trailing stop before testing levels, so a bar that both
+        # advances the trail and reverses is handled consistently.
+        if position.trailing_stop_pct and position.trailing_stop_pct > 0:
+            advanced = advance_trailing_stop(
+                position.side,
+                position.stop_loss,
+                position.trailing_stop_pct,
+                candle.high,
+                candle.low,
+            )
+            if advanced is not None and advanced != position.stop_loss:
+                position.stop_loss = advanced
+                self.store.update_stop_loss(position.id, advanced)
+
         hit = risk_exit(
             position.side,
             position.stop_loss,