feat: Add runtime config validation for cash, margin and memory settings

- Add initial_cash validation (must be positive number)
- Add margin_requirement validation (must be non-negative)
- Add enable_memory boolean configuration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/gateway.py

@@ -1236,6 +1236,48 @@ class Gateway:
                 return
             updates["max_comm_cycles"] = parsed_cycles
 
+        initial_cash = data.get("initial_cash")
+        if initial_cash is not None:
+            try:
+                parsed_initial_cash = float(initial_cash)
+            except (TypeError, ValueError):
+                parsed_initial_cash = 0.0
+            if parsed_initial_cash <= 0:
+                await websocket.send(
+                    json.dumps(
+                        {
+                            "type": "error",
+                            "message": "initial_cash must be a positive number.",
+                        },
+                        ensure_ascii=False,
+                    ),
+                )
+                return
+            updates["initial_cash"] = parsed_initial_cash
+
+        margin_requirement = data.get("margin_requirement")
+        if margin_requirement is not None:
+            try:
+                parsed_margin_requirement = float(margin_requirement)
+            except (TypeError, ValueError):
+                parsed_margin_requirement = -1.0
+            if parsed_margin_requirement < 0:
+                await websocket.send(
+                    json.dumps(
+                        {
+                            "type": "error",
+                            "message": "margin_requirement must be a non-negative number.",
+                        },
+                        ensure_ascii=False,
+                    ),
+                )
+                return
+            updates["margin_requirement"] = parsed_margin_requirement
+
+        enable_memory = data.get("enable_memory")
+        if enable_memory is not None:
+            updates["enable_memory"] = bool(enable_memory)
+
         if not updates:
             await websocket.send(
                 json.dumps(