With no SLACK_* secrets at all the app readiness check is NOT_CONFIGURED and
the health indicator reports UNKNOWN (HTTP 200) with configured=false, so an
optional, unset Slack integration can't fail a readiness probe (it took kof22's
probe down). Not-yet-checked is UNKNOWN too. Partial or invalid configuration
stays DOWN. Tests for all three.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>