[RELIABILITY][DVA PROVISIONING] Serialize concurrent virtual-account creation per user

[Obiajulu-gif]

Summary

Driver and investor DVA provisioning performs a read, several Paystack mutations, and only then an upsert. Concurrent requests can both create customers or dedicated accounts before MongoDB's uniqueness constraints arbitrate the final local record, leaking provider-side duplicates and ambiguous account routing.

Repository evidence

• lib/services/paystack-dva.service.ts:424-494 checks the local account before creating/updating a Paystack customer and dedicated account.
• lib/services/paystack-investor-dva.service.ts:333-394 repeats the same check-then-create sequence.
• models/DriverVirtualAccount.ts:97-99 and models/InvestorVirtualAccount.ts:90-92 enforce local uniqueness only after external side effects have happened.

Scope

• Add a per-user/provider provisioning claim with lease or fencing semantics before any Paystack mutation.
• Make retries resume or reconcile an in-flight attempt instead of creating another customer/account.
• Persist attempt state and provider identifiers early enough to recover ambiguous timeouts.

Acceptance criteria

• Concurrent requests for the same user produce one usable provider account and one canonical local record.
• A crashed or timed-out provision attempt can be safely resumed or reconciled.
• A stale worker cannot overwrite the result of a newer provisioning attempt.

Tests

• Concurrency tests for driver and investor requests, crash-after-customer creation, ambiguous DVA timeout, stale lease takeover, and duplicate-key recovery.

Non-goals

• Changing Paystack providers or redesigning wallet funding.

[Owolabenjade]

Hello maintainer, I want to work on this issue.