Adds coordinator.fleetMetrics() computing queue depth, stage histogram,
oldest-queued age (starvation signal), factory health and seat utilization,
plus derived alerts (no_live_capacity, all_factories_down, queue_starvation,
saturated, stale_factories). Exposed via GET /fleet/metrics and surfaced as a
metrics+alerts panel on the fleet overview. Thresholds injectable for tests.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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