Use locked exact-value FX quotes for down payments

[prissca]

What changed

• require a locked USD/NGN quote ID for down-payment initialization
• remove the live exchange-rate request and fallback rate from the payment mutation
• parse requested money into exact minor units and send the immutable quoted kobo amount to Paystack
• atomically consume locked quotes with a compare-and-set repository update
• attach quote source, timestamps, spread, exact input/output amounts, and consumer to payment metadata

Why

The payment route previously recomputed a floating JavaScript-number conversion at mutation time, bypassing quote expiry, replay, and audit controls.

Validation

• FX quote service tests: 7 passed, including concurrent consumption and exact-decimal boundaries
• changed-file typecheck scan: no errors in this diff; the repository-wide baseline still has unrelated failures

Closes #176

[Obiajulu-gif]

Thanks for the contribution! Merging this now as part of a maintainer PR sweep.