Dependency health check

Check that all upstream dependencies are reachable.

Tests connectivity to the Companies House API and Supabase database, and reports per-dependency status and response times. No authentication required.

Also reports api_self_check - not an upstream, but this API's own self-check probe: whether it is still running here, and what it last saw when it called GET /v1/company/{n} through the public hostname. An external uptime monitor watching this endpoint for the word unhealthy therefore also catches the case where our internal monitoring has stopped.

• healthy - all dependencies responding normally
• degraded - one or more dependencies are slow or unreachable (the API will still serve cached data)

Upstream usage: 1 upstream call to the Companies House API (a lightweight search probe). Do not poll this endpoint at high frequency - once per minute is sufficient for monitoring.