Re-validation cron for registered servers

[funkiirabu]

Problem

Once a server is registered, there's no periodic check that it's still alive, still has a valid OpenAPI spec, or still responds with 402 challenges. Servers could go offline and remain listed indefinitely.

Solution

Add a cron job that periodically re-validates registered servers.

Checks to run

• Server URL is reachable (HTTP 200 or 402)
• /openapi.json still exists and parses correctly
• Endpoint count matches (detect added/removed endpoints)
• 402 challenge still works on a sample endpoint
• Recipient address hasn't changed

Behavior

• Run daily (or every 6 hours)
• On failure: set server.status = 'unreachable' after N consecutive failures
• On recovery: automatically restore to active
• Update server.endpointData if endpoints changed
• Log validation results for debugging

Implementation

• Next.js API route: POST /api/cron/revalidate
• Protected by CRON_SECRET (same pattern as other crons)
• Uses @suimpp/discovery check internally

Context

Deferred from Phase 4 scope. Currently servers are validated once at registration and never rechecked.