Add optional intro message on bot startup

When SEND_INTRO_MESSAGE=true, the bot sends "The deals must flow." as
an m.notice to the Matrix room right after startup, before the first
scheduled deal checks. Defaults to off.

https://claude.ai/code/session_017eMsVwUopgmnEyd6JJedpV

gaming_deals_bot/__main__.py

@@ -69,6 +69,8 @@ async def main():
     scheduler.start()
     logger.info("Bot started — scheduler running")
 
+    await bot.send_intro()
+
     # Run initial checks immediately (after first-run population is done)
     await bot.check_cheapshark()
     await bot.check_epic_free_games()