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>

.env.example

@@ -1,6 +1,7 @@
 # Discord Configuration
 DISCORD_TOKEN=your_discord_bot_token_here
 CHANNEL_ID=your_channel_id_here,optional_second_channel_id
+STARTUP_ANNOUNCEMENT=
 
 # Bot Configuration
 COMMAND_PREFIX=!