fix: price API exchange code mapping and VTS overseas balance fallback (#147)

- Apply _PRICE_EXCHANGE_MAP in get_overseas_price() to send short codes
  (NASD→NAS, NYSE→NYS, AMEX→AMS) required by HHDFS00000300 price API
- Add PAPER_OVERSEAS_CASH config setting (default $50,000) for simulated
  USD balance when VTS overseas balance API returns 0 in paper mode
- Fall back to scan candidate price when live price API returns 0
- Both fixes together resolve "no affordable quantity (cash=0, price=0)"
  which was preventing all overseas trade execution

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

src/config.py

@@ -55,6 +55,11 @@ class Settings(BaseSettings):
     # Trading mode
     MODE: str = Field(default="paper", pattern="^(paper|live)$")
 
+    # Simulated USD cash for VTS (paper) overseas trading.
+    # KIS VTS overseas balance API returns errors for most accounts.
+    # This value is used as a fallback when the balance API returns 0 in paper mode.
+    PAPER_OVERSEAS_CASH: float = Field(default=50000.0, ge=0.0)
+
     # Trading frequency mode (daily = batch API calls, realtime = per-stock calls)
     TRADE_MODE: str = Field(default="daily", pattern="^(daily|realtime)$")
     DAILY_SESSIONS: int = Field(default=4, ge=1, le=10)