fix(scoring): consolidate duplicate user score fields in schema (#89)

[lucky-pluton]

Closes #89 — Part of the #87 scoring-ledger epic.

TL;DR

The User schema had overlapping/duplicate score fields (\coins\ vs \coinBalance, \xp\ vs \xpScore) that could drift out of sync. This PR consolidates all reads/writes onto canonical \coins\ and \xp\ fields, removes duplicate schema fields and the auto-synchronizing \pre('save')\ hook, and adds an idempotent migration script.

Changes

• Remove \coinBalance\ and \xpScore\ from \UserSchema\ and \UserStatsSchema\ in \server/auth.js.
• Remove \pre('save')\ mirroring hook from \UserSchema.
• Update all call sites (\server/hints/index.js, \server/lil/studentState.js) to read/write canonical \coins\ and \xp.
• Add \server/scripts/migrate-score-fields.js\ to migrate existing documents without data loss.
• Add \server/tests/migrateScoreFields.test.js\ unit tests for the migration script.

Verification

• 
  pm test\ in \server/: all tests passed.
• 
  px vitest run tests/migrateScoreFields.test.js: 8/8 tests passed.
• Grep test: \grep -rn 'coinBalance\|xpScore' server/ client/src\ returns no results outside the migration script.

[github-actions]

✅ Parikshak — all automated checks pass

• ✅ Linked to open issue Scoring: consolidate duplicate user score fields in the schema #89 — Scoring: consolidate duplicate user score fields in the schema
• ✅ No merge conflict

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