feat(scoring): implement atomic recordEvent() ledger writer (#90)

[lucky-pluton]

Closes #90 — Part of the #87 scoring-ledger epic.

TL;DR

Replaces lost-update read-modify-write patterns with a centralized
ecordEvent(userId, event)\ ledger writer that mutates user scores atomically using MongoDB \\ and \\ operators.

Changes

• Add \server/lib/ledger.js: exports
  ecordEvent(userId, event)\ which looks up reward calculations from \scoring.js\ and executes atomic \indOneAndUpdate\ updates.
• Add \server/tests/ledger.test.js: unit tests verifying reward calculations, debit operations, and high-concurrency atomic execution (10 parallel requests without lost updates).

Verification

• 
  pm test\ in \server/: all tests passed (191 tests).
• 
  px vitest run tests/ledger.test.js: 4/4 tests passed including concurrency test.

[github-actions]

✅ Parikshak — all automated checks pass

• ✅ Linked to open issue Scoring: implement atomic recordEvent() ledger writer #90 — Scoring: implement atomic recordEvent() ledger writer
• ✅ No merge conflict

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