feat(scoring): add central reward rule-book (#88)

[lucky-pluton]

Closes #88 — Part of the #87 scoring-ledger epic.

TL;DR

Consolidates reward/point rules scattered across server and client into a single pure, unit-testable rule-book (\server/lib/scoring.js).

Changes

• Add \server/lib/scoring.js: pure functions for quiz rewards, review completion, topic badges, collection completions, transfer wins, daily check-ins, and hint unlocks.
• Add \server/tests/scoring.test.js: comprehensive vitest tests for all rule-book calculations and immutability guarantees.

Verification

• 
  pm test\ in \server/: all tests passed.

[github-actions]

✅ Parikshak — all automated checks pass

• ✅ Linked to open issue Scoring: add a central reward rule-book (server/lib/scoring.js) #88 — Scoring: add a central reward rule-book (server/lib/scoring.js)
• ✅ No merge conflict

This is Parikshak, an automated, comment-only check — it never closes or approves a PR. A human reviewer still makes the final call. See CONTRIBUTING.md for the full contribution rules.