add missing index in repositories table, order updates by oldest first

[syphar]

• I saw the index was missing but the column was queried. With the exception of small tables, all columns used in WHERE statements should have an index.
• I want to update the repositories first that were updated the earliest.

current state from prod:

cratesfyi=> select min(updated_at), max(updated_at) from repositories;
              min              |              max
-------------------------------+-------------------------------
 2026-02-04 16:28:14.709552+00 | 2026-02-08 04:24:43.370584+00
(1 row)

( This is better than yesterday when I checked, so I think we're still making enough progress with updates, despite the rate limiting)

[GuillaumeGomez]

Thanks!