[Backend] — Match Score and Multiplier Application

[Olowodarey]

Problem
matches.service.ts stores score/multiplier fields but scoring doesn't consistently apply the multiplier when awarding points.

Requirements

• Apply the match multiplier when computing awarded points on result submission.
• Guard against retroactive multiplier changes after scoring.
• Tests: points reflect multiplier; post-score multiplier change is rejected.

Acceptance Criteria

• Match points correctly incorporate the configured multiplier. Covered by tests.

Files

• backend/src/matches/matches.service.ts
• backend/src/migrations/1775800000000-AddScoreAndMultiplierToMatch.ts
• backend/src/migrations/1775900000000-AddScorelineAndPointsToMatchPrediction.ts