chore: Switch to computed scores across the autopilot

Background

The recent combinatorial auction implementation highlighted the need to stop relying on scores provided by solvers and to start using our computed scores instead. While the combinatorial auction logic already relies solely on computed scores, other parts of the autopilot—such as the legacy MaxScore implementation and database storage—still use solver-provided scores.

Once the combinatorial auction cutover (#3387) is complete, we should fully transition to computed scores:

• Compute the score during construction of the Solution struct
• Use a single field for the score (currently, we have both Solution.score and Solution.computed_score)
• Remove the score from the solver API, so solvers no longer need to provide it
• Store the computed score in the database

Ref: https://github.com/cowprotocol/services/pull/3397#discussion_r2105811873

Start by tracing construction of the Solution struct, then inspect the solver API and database storage paths described in the issue. The work is done when the autopilot uses one computed score throughout, solvers no longer provide a score, and the computed value is stored in the database.