[FIX] runbot: ensure correct batch order in last_batchs compute

[vib-adhoc]

Problem:
In some environments, the last_batchs field was returning batches in a non-deterministic order, even though the backend query was intended to sort them by most recent first. This was due to the use of PostgreSQL's ARRAY_AGG function without an explicit ORDER BY clause, which does not guarantee the order of elements in the resulting array.

This caused the frontend to display batches in the wrong order, despite the backend shell showing them correctly, leading to confusion and inconsistent UI behavior.

Solution:
Explicitly specify the order in the ARRAY_AGG function within the SQL query used in _compute_last_batchs. By adding ORDER BY batch.id DESC, we ensure that the array of batch IDs is always sorted from newest to oldest, matching the intended logic and the backend results.

[vib-adhoc]

@d-fence Hello Christophe, could you please take a look at this PR?