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/broker/kis_api.py

@@ -6,11 +6,8 @@ Handles token refresh, rate limiting (leaky bucket), and hash key generation.
 from __future__ import annotations
 
 import asyncio
-import hashlib
-import json
 import logging
 import ssl
-import time
 from typing import Any
 
 import aiohttp
@@ -168,7 +165,7 @@ class KISBroker:
                         f"get_orderbook failed ({resp.status}): {text}"
                     )
                 return await resp.json()
-        except (aiohttp.ClientError, asyncio.TimeoutError) as exc:
+        except (TimeoutError, aiohttp.ClientError) as exc:
             raise ConnectionError(f"Network error fetching orderbook: {exc}") from exc
 
     async def get_balance(self) -> dict[str, Any]:
@@ -200,7 +197,7 @@ class KISBroker:
                         f"get_balance failed ({resp.status}): {text}"
                     )
                 return await resp.json()
-        except (aiohttp.ClientError, asyncio.TimeoutError) as exc:
+        except (TimeoutError, aiohttp.ClientError) as exc:
             raise ConnectionError(f"Network error fetching balance: {exc}") from exc
 
     async def send_order(
@@ -253,5 +250,5 @@ class KISBroker:
                     },
                 )
                 return data
-        except (aiohttp.ClientError, asyncio.TimeoutError) as exc:
+        except (TimeoutError, aiohttp.ClientError) as exc:
             raise ConnectionError(f"Network error sending order: {exc}") from exc