DB-based solver participation guard query fix (#3305)

In one of the previous PRs, I removed the ordering which actually broke
the logic.
#3263 (comment)
I actually can't remember why I decided to do so...

crates/database/src/solver_competition.rs

@@ -174,6 +174,7 @@ WITH
             SELECT DISTINCT ca.id AS auction_id
             FROM competition_auctions ca
             WHERE ca.deadline <= $1
+            ORDER BY ca.id DESC
             LIMIT $2
         ) latest_auctions
         JOIN proposed_solutions ps ON ps.auction_id = latest_auctions.auction_id