UX: Surface health information on registry pages

Problem

Platform engineers need a quick way to assess the overall upstream health of a registry. A registry can contain multiple upstreams, so a roll-up metric helps spot issues at a glance without scanning each upstream.

Solution

Surface registry health on the Registries list and Registry detail page.

Out of scope for this issue, but considered for future iterations, is expanding the definition of "health" to include operational signals such as response time, cache hit ratio, and total uptime.

Design Requirements

• Make upstream health status immediately understandable at a glance
• Use clear visual indicators (e.g., icons, colours, text) to convey connectivity status
• Ensure placement supports quick discovery without disrupting existing workflows
• Consider both high-level overview and paths to more detailed information

This issue is related to UX: Surface health information on upstream pages (#560911). It is recommended to complete that upstream health implementation before working on registry health, since the roll-up depends on it.