feat: implement TelegramClient core module (issue #31)

Add TelegramClient for real-time trading notifications:
- NotificationPriority enum (LOW/MEDIUM/HIGH/CRITICAL)
- LeakyBucket rate limiter (1 msg/sec)
- 8 notification methods (trade, circuit breaker, fat finger, market open/close, system start/shutdown, errors)
- Graceful degradation (optional API, never crashes)
- Session management pattern from KISBroker
- Comprehensive README with setup guide and troubleshooting

Follows NewsAPI pattern for optional APIs.
Uses existing aiohttp dependency.

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

src/notifications/__init__.py

@@ -0,0 +1,5 @@
+"""Real-time notifications for trading events."""
+
+from src.notifications.telegram_client import TelegramClient
+
+__all__ = ["TelegramClient"]