[FE-38] Build /admin/providers page — external validation provider health status cards

[mftee]

Overview

Once external validation providers are real (BE-50/BE-51/BE-52), administrators need to monitor whether each provider is healthy and reachable. Downtime in a validation provider means documents may stall in external validation. The providers page shows the current health of each integration.

Background

File to create: frontend/app/(protected)/admin/providers/page.tsx

Access: Admin role only

Features:

• One card per provider: Land Registry, Government ID, Business Registration
• Each card shows:
  • Provider name and logo/icon
  • Health status: Online (green) / Degraded (yellow) / Offline (red)
  • Last health check timestamp ("Checked 2 minutes ago")
  • Success rate (last 24 hours): e.g., "94% success"
  • Average response time: e.g., "213ms"
• "Run health check now" button per card → calls GET /api/external-validation/health
• Auto-refresh every 60 seconds

Acceptance Criteria

• Cards fetch data from GET /api/external-validation/stats (BE-53)
• "Run health check now" calls the health endpoint and updates the card in real time
• Offline status shown prominently in red with a warning icon
• Auto-refresh timer visible (countdown to next refresh)
• Page shows loading skeleton while first fetch is in progress