[HARD][FX] Route down-payment conversion through signed, exact-value quote snapshots

[Obiajulu-gif]

Summary

The down-payment route fetches a floating USD/NGN rate directly and converts JavaScript numbers inside the payment request, bypassing the repository-backed quote lifecycle and its expiry/audit policies.

Repository evidence

• app/api/payments/down-payment/route.ts calls api.exchangerate-api.com directly and returns a number rate.
• The repository already contains lib/fx/quote-service.ts for explicit quote snapshots and consumption.

Scope

• Require a locked quote ID for cross-currency down payments.
• Use exact minor-unit arithmetic and atomically consume the quote when initializing Paystack.
• Record rate source, timestamp, spread, input/output amounts, and consumer.

Acceptance criteria

• No live floating rate is fetched inside the payment mutation.
• The amount sent to Paystack exactly matches the consumed quote snapshot.
• Expired/reused/mismatched quotes fail without creating a payment.

Tests

• Exact rounding tests for USD/NGN boundaries.
• Integration tests for expiry, replay, concurrent consumption, and provider failure.

Non-goals

• Changing the supported currency pair or FX provider roster.

[prissca]

i will love to work on this issue

[grantfox-oss]

🦊 GrantFox — @prissca has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #176)
2. Your PR will be reviewed by the Chainmove maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@prissca's PR #208 was approved and merged by @Obiajulu-gif.

🏆 @prissca: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,262 total points). Track your full progress on GrantFox.

👏 Great work, @prissca! Keep contributing to Chainmove.