feat(api-gateway): add fx-engine HTTP client for payment quotes

[DANTE-1903]

Summary

Closes #114

Implements an HTTP client within api-gateway to integrate seamlessly with fx-engine. Payment creation flows now support fetching automated currency conversion quotes when the optional convertTo parameter is supplied.

What Changed

• Created services/api-gateway/src/clients/fx-client.ts implementing getQuote with a strict 5-second timeout, x-service-token authentication propagation, and structured metric/error logging.
• Created services/api-gateway/src/clients/fx-client.test.ts providing 18 robust test variations covering happy paths, timeout scenarios, network drops, and fallback behaviors.
• Modified prisma/schema.prisma to include core FX fields (convertTo, convertedAmount, fxRate, fxQuoteId, fxQuoteExpiresAt) directly on the Payment model.
• Modified shared/validation/schemas.ts to cleanly validate the incoming optional convertTo payload attribute.
• Modified services/api-gateway/src/index.ts to orchestrate client execution gracefully (ensuring payments proceed unhindered if the fx-engine undergoes an outage).

Checklist

• Create an HTTP client in api-gateway that calls the fx-engine
• On payment creation, optionally fetch a quote if convertTo parameter is provided
• Pass the merchant's JWT or a service token for inter-service auth
• Handle fx-engine unavailability gracefully (timeout, fallback)
• Log inter-service call metrics
• Ensure full typecheck (tsc --noEmit) and all tests pass cleanly

Security & Resilience Note

Inter-service calls explicitly enforce a strict 5-second fallback ceiling. If the downstream fx-engine is unavailable, timeouts or errors are caught safely, falling back gracefully to log metrics and allow local payment transactions to process normally without cascading failures.

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[therealjhay]

Kindly resolve conflict