fix: overseas order rt_cd check, limit price premium, paper cash fallback (#151)

Three fixes for overseas stock trading failures:

1. Price API exchange code mapping:
   - get_overseas_price() now applies _PRICE_EXCHANGE_MAP (NASD→NAS, NYSE→NYS, AMEX→AMS)
   - Price API HHDFS00000300 requires short exchange codes same as ranking API

2. rt_cd check in send_overseas_order():
   - Log WARNING (not INFO) when rt_cd != "0" (e.g., "주문가능금액이 부족합니다")
   - Caller (main.py) checks rt_cd == "0" before calling log_trade()
   - Prevents DB from recording failed orders as successful trades

3. Limit order price premium for BUY:
   - BUY limit price = current_price * 1.005 (0.5% premium)
   - SELL limit price = current_price (no premium)
   - Improves fill probability: KIS VTS only accepts limit orders,
     and last price is typically at or below ask

4. PAPER_OVERSEAS_CASH fallback (config + main.py):
   - New setting: PAPER_OVERSEAS_CASH = 50000.0 (USD)
   - When VTS overseas balance API fails/returns 0, use this as simulated cash
   - Applied in both trading_cycle() and run_daily_session()

5. Candidate price fallback:
   - If price API returns 0, use scanner candidate price as fallback

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

src/broker/overseas.py

@@ -257,14 +257,27 @@ class OverseasBroker:
                         f"send_overseas_order failed ({resp.status}): {text}"
                     )
                 data = await resp.json()
-                logger.info(
-                    "Overseas order submitted",
-                    extra={
-                        "exchange": exchange_code,
-                        "stock_code": stock_code,
-                        "action": order_type,
-                    },
-                )
+                rt_cd = data.get("rt_cd", "")
+                msg1 = data.get("msg1", "")
+                if rt_cd == "0":
+                    logger.info(
+                        "Overseas order submitted",
+                        extra={
+                            "exchange": exchange_code,
+                            "stock_code": stock_code,
+                            "action": order_type,
+                        },
+                    )
+                else:
+                    logger.warning(
+                        "Overseas order rejected (rt_cd=%s): %s [%s %s %s qty=%d]",
+                        rt_cd,
+                        msg1,
+                        order_type,
+                        stock_code,
+                        exchange_code,
+                        quantity,
+                    )
                 return data
         except (TimeoutError, aiohttp.ClientError) as exc:
             raise ConnectionError(