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/strategies/base.py

@@ -26,6 +26,7 @@ class Intent:
     size: float | None = None
     stop_loss: float | None = None
     take_profit: float | None = None
+    trailing_stop_pct: float = 0.0
     reason: str = ""
 
 
@@ -121,18 +122,24 @@ class StrategyContext:
         size: float | None = None,
         stop_loss: float | None = None,
         take_profit: float | None = None,
+        trailing_stop_pct: float = 0.0,
         reason: str = "",
     ) -> None:
-        self._intents.append(Intent("enter_long", size, stop_loss, take_profit, reason))
+        self._intents.append(
+            Intent("enter_long", size, stop_loss, take_profit, trailing_stop_pct, reason)
+        )
 
     def enter_short(
         self,
         size: float | None = None,
         stop_loss: float | None = None,
         take_profit: float | None = None,
+        trailing_stop_pct: float = 0.0,
         reason: str = "",
     ) -> None:
-        self._intents.append(Intent("enter_short", size, stop_loss, take_profit, reason))
+        self._intents.append(
+            Intent("enter_short", size, stop_loss, take_profit, trailing_stop_pct, reason)
+        )
 
     def close(self, reason: str = "") -> None:
         self._intents.append(Intent("close", reason=reason))