fix: apply bulk score deltas in correct direction

[silentgeckoaudit3801]

Fixes #1318.

Updates �ackend/src/services/scoresService.ts so updateUserScoresBulk builds inserted/upserted score payloads with 500 + delta. That keeps positive deltas as rewards and negative deltas as penalties when the conflict update computes scores.current_score + EXCLUDED.current_score - 500.

Adds a regression in �ackend/src/services/tests/scoresService-additions.test.ts that locks the positive-delta SQL shape and rejects the old subtractive payload.

Validation: static source/API inspection only; local backend tests were not run because this environment is avoiding dependency/toolchain installation or execution.

[ogazboiz]

good catch, the insert path really did apply positive deltas as penalties (500 - delta vs the upsert's + EXCLUDED math), and 500 + delta lines both paths up. one tiny blocker: CI lint fails on a missing trailing newline in backend/src/services/tests/scoresService-additions.test.ts (prettier at 94:6). run:

cd backend && npx prettier --write src/services/__tests__/scoresService-additions.test.ts && git commit -am "chore: prettier" && git push

and this merges.

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