Add fleet-wide health aggregation

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Problem: The existing readiness check is per-instance only — there's no fleet-wide health aggregation for an operator centrally managing many instances.

Area: ORB / Incident readiness

Proposal: Build a fleet-wide health aggregation view on top of the existing per-instance check.

Deliverables:

• A fleet-wide health dashboard/aggregation.

AMS Cloud Readiness cross-reference (added 2026-07-12)

See also #5226 in the new AMS Cloud Readiness milestone (#28) — the AMS-side parallel to this exact problem, already citing this issue as precedent. Keep both sides in sync if either design changes.

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JSO-3592

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Once #7177 (AMS's new health endpoint) lands, this aggregation should likely cover both ORB and AMS hosted containers from one dashboard rather than being ORB-only — see #7173 and #5226.