Remove redundant health check route

[kleutzinger]

Summary

Fixes deployment failures caused by duplicate route registration for /api/health. PocketBase has included a built-in /api/health endpoint since v0.10.0, making the custom hook redundant and causing route
conflicts.

Problem

When deploying to Dokku, the application would panic on startup with:
panic: pattern "GET /api/health" conflicts with pattern "GET /api/health"

This occurred because:

1. PocketBase already provides /api/health (since v0.10.0)
2. The custom pb_hooks/health.pb.js was attempting to register the same route
3. Go 1.22+ enforces stricter route conflict detection, causing the panic

Changes

• Deleted pb_hooks/health.pb.js: Removed custom health endpoint that duplicated built-in functionality
• Updated CHECKS: Simplified health check syntax to verify the built-in endpoint returns "API is healthy" in the response

Before:
/api/health HTTP/1.1
Host: pocketbase
User-Agent: Dokku/1.0

After:
/api/health API is healthy

Result

• ✅ Deployment completes successfully
• ✅ Health checks pass
• ✅ Uses PocketBase's built-in, maintained endpoint instead of custom code