[Easy] Make legacy solver competition data parseable (#2209)

# Description
We've been seeing error message alerts when loading very competition
data from the db. This is expected (cf. #2143) however it is pertubating
on-call. The alternative would be to downgrade the warning to an error,
however I feel we may be losing signal of future breakages in this case.

# Changes
<!-- List of detailed changes (how the change is accomplished) -->

- [x] use serde[(default)] where possible
- [x] for score, which is a flattened enum that doesn't support defaults
(cf. this [open issue](serde-rs/serde#1626)),
we go back to making it an optional. This is fine since with
rank-by-surplus we likely no longer need mandatory score anymore
anyways.

## How to test
Unit test for old transactions data

crates/autopilot/src/run_loop.rs

@@ -239,7 +239,7 @@ impl RunLoop {
                         let mut settlement = SolverSettlement {
                             solver: participant.driver.name.clone(),
                             solver_address: participant.solution.account,
-                            score: Score::Solver(participant.solution.score.get()),
+                            score: Some(Score::Solver(participant.solution.score.get())),
                             ranking: solutions.len() - index,
                             orders: participant
                                 .solution