[Easy] Make legacy solver competition data parseable

[fleupold]

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

• use serde[(default)] where possible
• for score, which is a flattened enum that doesn't support defaults (cf. this open issue), 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