feat: implement timezone-based global market auto-selection

Implement comprehensive multi-market trading system with automatic
market selection based on timezone and trading hours.

## New Features
- Market schedule module with 10 global markets (KR, US, JP, HK, CN, VN)
- Overseas broker for KIS API international stock trading
- Automatic market detection based on current time and timezone
- Next market open waiting logic when all markets closed
- ConnectionError retry with exponential backoff (max 3 attempts)

## Architecture Changes
- Market-aware trading cycle with domestic/overseas broker routing
- Market context in AI prompts for better decision making
- Database schema extended with market and exchange_code columns
- Config setting ENABLED_MARKETS for market selection

## Testing
- 19 new tests for market schedule (timezone, DST, lunch breaks)
- All 54 tests passing
- Lint fixes with ruff

## Files Added
- src/markets/schedule.py - Market schedule and timezone logic
- src/broker/overseas.py - KIS overseas stock API client
- tests/test_market_schedule.py - Market schedule test suite

## Files Modified
- src/main.py - Multi-market main loop with retry logic
- src/config.py - ENABLED_MARKETS setting
- src/db.py - market/exchange_code columns with migration
- src/brain/gemini_client.py - Dynamic market context in prompts

Resolves #5

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/logging_config.py

@@ -2,20 +2,19 @@
 
 from __future__ import annotations
 
+import json
 import logging
 import sys
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from typing import Any
 
-import json
-
 
 class JSONFormatter(logging.Formatter):
     """Emit log records as single-line JSON objects."""
 
     def format(self, record: logging.LogRecord) -> str:
         log_entry: dict[str, Any] = {
-            "timestamp": datetime.now(timezone.utc).isoformat(),
+            "timestamp": datetime.now(UTC).isoformat(),
             "level": record.levelname,
             "logger": record.name,
             "message": record.getMessage(),