feat: add background memory pruning task

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bot.py

@@ -209,6 +209,22 @@ class BCSBot(commands.Bot):
                     ", ".join(missing),
                 )
 
+        # Start memory pruning background task
+        if not hasattr(self, "_memory_prune_task") or self._memory_prune_task.done():
+            self._memory_prune_task = asyncio.create_task(self._prune_memories_loop())
+
+    async def _prune_memories_loop(self):
+        """Background task that prunes expired memories every 6 hours."""
+        await self.wait_until_ready()
+        while not self.is_closed():
+            try:
+                count = await self.db.prune_expired_memories()
+                if count > 0:
+                    logger.info("Pruned %d expired memories.", count)
+            except Exception:
+                logger.exception("Memory pruning error")
+            await asyncio.sleep(6 * 3600)  # Every 6 hours
+
     async def close(self):
         await self.db.close()
         await self.llm.close()