v5.4.3: Lower background queue job fetch limit

Reduces the maximum number of background jobs fetched and processed
per iteration from 10,000 to 1,000.

While the previous increase to 10,000 aimed to reduce database round-trips,
processing such a large batch concurrently could still lead to excessive
memory consumption for specific job types, resulting in out-of-memory
errors. This adjustment provides a better balance between database
efficiency and peak memory usage during job execution.