[Admin] Admin dashboard overview metrics

[Xaxxoo]

Overview

Real-time platform overview for admins: key metrics and system health.

Tasks

• GET /api/v1/admin/stats endpoint
• Metrics: total merchants, total payments, total settled volume (USD)
• Active payments in last 24 hours
• System health: DB, Stellar, partner API status

Acceptance Criteria

• Stats computed from live DB (cached 30s)
• Stellar connectivity status included
• Response < 200ms with cache hit