[HARD][FINANCE] Implement an immutable double-entry ledger for all balance movements (#81)

[edehvictor]

Overview

Introduces an immutable double-entry accounting ledger as the single source of truth for ChainMove wallet, investment, repayment, payout, fee, and adjustment balances.

Summary of Changes

1. Ledger Models (models/LedgerAccount.ts, models/LedgerJournal.ts, models/LedgerEntry.ts):

   • Defined \LedgerAccount\ covering investor wallets, driver balances, pool escrow, platform clearing, revenue/fees, repayments receivable, payouts payable, and adjustment accounts.
   • Defined \LedgerJournal\ with unique
     eferenceKey\ for idempotency, event types, reversal tracking, and Mongoose hooks enforcing immutability.
   • Defined \LedgerEntry\ for explicit debit and credit entries with currency enforcement and immutability controls.

2. Posting Service (lib/ledger/posting.service.ts):

   • Enforces system-wide balance validation (debits == credits per currency).
   • Domain helpers: \postWalletFunding, \postWalletDebit, \postPoolInvestment, \postDownPayment, \postRepayment, \postRefund, \postPayout, \postFee, and \postAdjustment.
   • Balanced reversal mechanism (
     everseJournal) preserving audit trails.

3. Projection & Reconciliation (lib/ledger/projection.service.ts):

   • \computeAccountBalance: Calculates net balance according to standard accounting rules.
   • 
     ebuildUserBalances: Syncs cached \User.availableBalance\ from ledger entries.
   • 
     econcileLedger: Audits system-wide invariant (sum of debits == sum of credits per currency).

4. Backfill Migration (scripts/migrations/backfill-ledger.ts):

   • Migration script supporting --dry-run\ and --resume\ flags for backfilling legacy transactions.

5. Test Suite (tests/lib/ledger.test.ts):

   • Unit tests covering journal balancing, idempotency, reversals, projection rebuilds, and reconciliation.

Fixes #81