feat(backend): add on-chain portfolio reconciliation and indexer repair jobs

[Ibinola]

Summary

Implements an idempotent on-chain portfolio reconciliation process and BullMQ indexer repair jobs for BACKit on Stellar.

Closes #562

Key Implementation Details

1. Reconciliation Service & BullMQ Job: Built ReconciliationService and ReconciliationProcessor (QUEUE_RECONCILIATION) scanning configurable ledger ranges (fromLedger to toLedger) and contract IDs.
2. 4 Record Types: Compares call creation, stake, resolution, and payout claim events against PostgreSQL using stable event identities (contractId:ledger:txHash:eventIndex).
3. Discrepancy Classification: Classifies detected discrepancies into 5 categories: MISSING_OFFCHAIN, DUPLICATE_OFFCHAIN, VALUE_MISMATCH, UNKNOWN_CONTRACT, and UNRECOVERABLE.
4. Dry-Run & Repair Modes:
   • Dry-run mode: Produces structured report without modifying database records.
   • Repair mode: Transactionally upserts missing records and updates statuses without ever submitting Soroban transactions. Quarantines unsafe mismatches into ReconciliationDiscrepancy.
5. Idempotency & Distributed Locking: Enforces distributed lock (reconciliation_lock:<network>) to prevent overlapping runs and guarantees idempotent re-runs over identical ranges.
6. Protected Admin Endpoints: Added POST /admin/reconciliation/run, GET /admin/reconciliation/runs, GET /admin/reconciliation/runs/:id, and GET /admin/reconciliation/discrepancies.
7. Operator Runbook & Test Suite: Added docs/RECONCILIATION_RUNBOOK.md and comprehensive unit test suite in reconciliation.service.spec.ts.

[PeterOche]

lgtm