feat: strategy builder

backend/routers/strategy_builder.py

@@ -320,6 +320,11 @@ def price(req: PriceRequest):
                 "max_gain": result.get("max_gain"), "max_loss": result.get("max_loss"),
                 "bounded_risk": bounded_risk,
             },
+            # Exact per-leg quote (exec_price/mid) actually used to build entry_cost — lets
+            # entry_cost be reconciled by hand against THIS number, instead of whatever the
+            # leg dropdown's own (independently-fetched, possibly a few seconds older/newer)
+            # chain snapshot happens to show at the moment of reading the screen.
+            "leg_prices": result.get("leg_prices"),
             "risk_debug": result.get("risk_debug"),
         },
     )