telemetry: confirmed-cluster stats
Count a cluster as confirmed once it has reported on >=2 distinct UTC
days (per-cluster history makes this a length check). Version/OS
distributions in /api/stats are computed over confirmed clusters, so a
one-shot injected report can't appear in them; falls back to all active
clusters while no confirmed ones exist (fresh server). Adds the
seaweedfs_telemetry_confirmed_clusters gauge and a dashboard card.
