Add FX rate staleness monitoring and admin visibility

[afeezorobsco-cyber]

##close #332

Summary

This PR adds observability for stale FX rates by introducing configurable staleness thresholds, structured logging, and per-currency-pair staleness reporting in the admin status endpoint.

Changes

Configuration

Added a new environment variable:

Variable	Default	Description
MAX_STALE_SECONDS	300	Maximum acceptable age of a served FX rate before it is considered stale.

Staleness Monitoring

When serving a rate:

• Calculates the rate age in seconds.
• Compares it with MAX_STALE_SECONDS.

Logging behaviour:

• Fresh rate → no additional log.
• Stale live rate → structured WARN log.
• Stale seed/fallback rate → structured ERROR log.

The change is observational only and does not alter pricing or request handling.

Admin Status

Extended the admin status endpoint to expose per-currency-pair staleness, including:

• Currency pair
• Rate source
• Last update timestamp
• stalenessSeconds
• stale flag

Testing

Added coverage for:

• ✅ Fresh rate (no warning).
• ✅ Stale live rate (WARN).
• ✅ Stale seed rate (ERROR).
• ✅ Boundary at MAX_STALE_SECONDS.
• ✅ Correct staleness values in the admin status endpoint.

Verification

Executed:

npm test
npm run lint
npm run build

Verified that:

• Fresh rates do not generate alerts.
• Stale live rates emit WARN logs.
• Stale seed/fallback rates emit ERROR logs.
• The admin status endpoint accurately reports per-pair staleness.
• Existing FX behaviour remains unchanged.

Closes #

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[afeezorobsco-cyber]

@therealjhay pls review and merge