Console and build-log relays hold a Server-Sent Event connection open for the
life of a client's attachment; a stalled reader pins the relay goroutine plus
its upstream kube-apiserver follow. Without a bound, one authenticated
principal could open these repeatedly and accumulate leaked control-plane
connections.
Add a per-principal stream cap (streamLimiter) enforced before either relay
opens its follow stream, returning 429 too_many_streams past the limit.
cmd/felis wires it to 16; zero disables it, matching the "zero disables"
idiom of the other levers.
This bounds the blast radius of the stalled-stream leak; it does not close the
leak itself -- the per-write deadline that severs a stalled stream is a
separate change.
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