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>

tests/test_main.py

@@ -805,13 +805,14 @@ class TestOverseasBalanceParsing:
                 scan_candidates={},
             )
 
-        # Verify limit order was sent with actual price, not 0.0
+        # Verify limit order was sent with actual price + 0.5% premium (issue #151), not 0.0
         mock_overseas_broker_with_buy_scenario.send_overseas_order.assert_called_once()
         call_kwargs = mock_overseas_broker_with_buy_scenario.send_overseas_order.call_args
         sent_price = call_kwargs[1].get("price") or call_kwargs[0][4]
-        assert sent_price == 182.5, (
-            f"Expected limit price 182.5 but got {sent_price}. "
-            "KIS VTS only accepts limit orders for overseas paper trading."
+        expected_price = round(182.5 * 1.005, 4)  # 0.5% premium for BUY limit orders
+        assert sent_price == expected_price, (
+            f"Expected limit price {expected_price} (182.5 * 1.005) but got {sent_price}. "
+            "KIS VTS only accepts limit orders; BUY uses 0.5% premium to improve fill rate."
         )