Fix/settlement finality unification 146

[Alaka-ibr]

Summary

Closes #146 by unifying settlement finality states across Paystack payments, bank transfers, internal ledger entries, and Stellar transactions into a canonical state machine.

Key Changes

• Canonical State Machine: Added SettlementRecord model and transition engine covering initiated, provider-pending, observed, provisionally_credited, confirmed, reversed, disputed, failed, and expired states.
• Balance Bucket Separation: Extended User schema with distinct availableBalance, pendingBalance, heldBalance, and reversedBalance fields so payout/investment operations depend strictly on confirmed funds.
• Reversal Journals: Standardized reversals through counter-debit journal entries rather than in-place edits, preserving complete audit trails without creating negative available balances.
• Causal Links & Idempotency: Established 1:1 cross-rail correlation across provider references, Stellar hashes, ledger journals, pool investments, and user transactions.
• Operator Timeline & API: Added operator timeline diagnostics and administrative remediation endpoints at /api/admin/settlement/timeline.

Verification

• Added test suite in __tests__/lib/settlement/settlement.test.ts covering out-of-order webhooks, delayed transfers, Stellar failures, reversals, duplicate references, timeouts, and concurrent processing.