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/models.py

@@ -67,6 +67,7 @@ class Position(Base):
     )
     stop_loss: Mapped[float | None] = mapped_column(Float, nullable=True)
     take_profit: Mapped[float | None] = mapped_column(Float, nullable=True)
+    trailing_stop_pct: Mapped[float] = mapped_column(Float, default=0.0)
     entry_fee: Mapped[float] = mapped_column(Float, default=0.0)
     exit_price: Mapped[float | None] = mapped_column(Float, nullable=True)
     exit_fee: Mapped[float] = mapped_column(Float, default=0.0)