Add optional startup announcement feature

Bot can now send a one-time announcement to all configured channels when it starts.

Configuration:
- Set STARTUP_ANNOUNCEMENT in .env with your message
- Leave empty to disable announcements
- Prevents duplicate announcements on reconnects

Usage example:
STARTUP_ANNOUNCEMENT=Bot updated! New features: colored prices and charts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

config.py

@@ -12,6 +12,7 @@ class Config:
     CHANNEL_ID = os.getenv('CHANNEL_ID')
     CHANNEL_IDS = [id.strip() for id in os.getenv('CHANNEL_ID', '').split(',') if id.strip()]
     COMMAND_PREFIX = os.getenv('COMMAND_PREFIX', '!')
+    STARTUP_ANNOUNCEMENT = os.getenv('STARTUP_ANNOUNCEMENT', '')
 
     # Stock configuration
     PRIMARY_TICKER = os.getenv('PRIMARY_TICKER', 'PYPL')