A 10-year leaf is pointless given the leaf auto-rotates (re-signed
within 30 days of expiry) and is long enough to make browsers warn.
397 days sits just under the 398-day CA/Browser-Forum leaf cap (so
browsers accept it) while bounding the blast radius of a leaf-key
compromise. The re-sign runs at service activation (hive-tls-ca is a
oneshot, no timer), so 397 stays comfortably above any gateway
rebuild gap.
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