Add backend metrics for payout queue depth and latency

[mimijuwonlo-commits]

Area

Backend observability

Problem

Operators need early signals when payout processing backs up. Queue depth, processing latency, and failure counts should be observable.

Acceptance Criteria

• Expose metrics for pending payout count, age, processing duration, and failure outcome
• Add tests around metric emission
• Avoid high-cardinality labels
• Document alert thresholds

Validation

Run backend metrics tests and update observability runbook.