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/evolution/optimizer.py

@@ -14,7 +14,7 @@ import logging
 import sqlite3
 import subprocess
 import textwrap
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from pathlib import Path
 from typing import Any
 
@@ -136,7 +136,7 @@ class EvolutionOptimizer:
             body = "\n".join(lines[1:-1])
 
         # Create strategy file
-        timestamp = datetime.now(timezone.utc).strftime("%Y%m%d_%H%M%S")
+        timestamp = datetime.now(UTC).strftime("%Y%m%d_%H%M%S")
         version = f"v{timestamp}"
         class_name = f"Strategy_{version}"
         file_name = f"{version}_evolved.py"
@@ -149,7 +149,7 @@ class EvolutionOptimizer:
 
         content = STRATEGY_TEMPLATE.format(
             name=version,
-            timestamp=datetime.now(timezone.utc).isoformat(),
+            timestamp=datetime.now(UTC).isoformat(),
             rationale="Auto-evolved from failure analysis",
             class_name=class_name,
             body=indented_body.strip(),