fix: backtest skips duplicate ticker positions to prevent overwrite

When multiple recommendations for the same ticker produce 'act'
decisions, the second one would overwrite the first in
simulated_positions, losing the first position's value and causing
incorrect portfolio value calculations. Now skips if already holding.

services/trading/backtest_replay.py

@@ -176,7 +176,7 @@ class BacktestReplay:
                         price = rec.get("current_price", 0.0)
                         qty = decision.computed_share_quantity or 0
 
-                        if qty > 0 and price > 0:
+                        if qty > 0 and price > 0 and ticker not in simulated_positions:
                             cost = price * qty
                             if cost <= portfolio_state.active_pool:
                                 simulated_positions[ticker] = {