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/config.py

@@ -19,6 +19,11 @@ class Config:
         self.min_discount_percent = int(os.environ.get("MIN_DISCOUNT_PERCENT", "50"))
         self.max_price_usd = float(os.environ.get("MAX_PRICE_USD", "20"))
 
+        # Intro message on startup
+        self.send_intro_message = os.environ.get(
+            "SEND_INTRO_MESSAGE", "false"
+        ).lower() in ("true", "1", "yes")
+
         # Database
         self.database_path = os.environ.get("DATABASE_PATH", "deals.db")