fix: save portfolio data to database

Portfolio (cash balance, position, etc.) is now saved to DB
during simulation so it persists and shows in frontend.

src/backend/app/db/models.py

@@ -95,6 +95,7 @@ class Simulation(Base):
     signals = Column(JSON)
     klines = Column(JSON)  # Price data for chart display
     trade_log = Column(JSON)  # Trade activity log
+    portfolio = Column(JSON)  # Portfolio data
 
     bot = relationship("Bot", back_populates="simulations")