fix: wrap synchronous _execute call in asyncio.to_thread for non-blocking mapper execution

src/snek/system/mapper.py

@@ -33,9 +33,8 @@ class BaseMapper:
                 self.db.commit()
             return result 
         
-        result = _execute()
         async with self.semaphore:
-            return _execute()
+            return await asyncio.to_thread(_execute)
 
     async def new(self):
         return self.model_class(mapper=self, app=self.app)