Multi-inbound clients showed online on every inbound they were attached to. Xray's user-level traffic stat aggregates across all inbounds a client belongs to, so the email signal alone can't say which inbound was used.
Pair it with the inbound-level traffic signal under the same 20s grace and gate the per-inbound rollup on it: a client only shows online on inbounds that actually moved bytes this window. Remote nodes report no per-inbound activity and stay ungated (no regression). Adds GetActiveInboundsByNode, the activeInbounds WS field and POST /panel/api/clients/activeInbounds.
Fixes#4859