Add presence heartbeat to keep bot shown as online

Adds a background asyncio.Task that calls set_presence("online") every
60 seconds to keep the homeserver presence status fresh. On startup the
bot sets itself online and spawns the heartbeat; on shutdown the task is
cancelled and presence is set to offline before closing the client.

https://claude.ai/code/session_01Nu3N2okrfJV3meatEmYdrB

gaming_deals_bot/bot.py

@@ -47,6 +47,9 @@ class DealsBot:
         else:
             self._first_run_done = True
 
+        # Start presence heartbeat so the bot shows as "online"
+        await self.matrix.start_presence_heartbeat()
+
     async def stop(self):
         """Clean shutdown."""
         await self._http.aclose()