fx-engine quote handler computes rates twice (cache + live fallback)

[therealjhay]

ProblemThe quote handler reads the cache and, on miss, fetches live; two paths recompute amounts and can return different results for the same request.

1. Duplicated quote computation.
2. Live-fallback path untested in fx-quote-* tests.
3. Inconsistent rounding across paths.

Solution

• Single quote computation path with cache-or-live resolution.
• Test the fallback path.
• Unify rounding.

Acceptance Criteria

• One computation path.
• Fallback tested.
• pnpm --filter fx-engine build passes.

Note for Contributors: Write a clear PR description. Show identical results from cache and fallback.

[TechBroAfrica]

Focused on building trust through consistent, meaningful contributions. I approach every issue with curiosity, attention to detail, and a commitment to delivering clean, maintainable, and well-tested solutions. Driven by impact, not just commits, I value clear communication, effective collaboration with maintainers, continuous learning, and delivering meaningful value to every project I contribute to.

[Andreschuks101]

Hi! I'd like to contribute by resolving this issue. I've gone through the requirements and am confident I can implement the requested changes while adhering to the project's conventions.

[grantfox-oss]

🦊 GrantFox — @Andreschuks101 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 #566)
2. Your PR will be reviewed by the Betta-Pay 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!

@Andreschuks101's PR #579 was approved and merged by @therealjhay.

🏆 @Andreschuks101: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (424 total points). Track your full progress on GrantFox.

👏 Great work, @Andreschuks101! Keep contributing to Betta-Pay.