fix(online): scope per-inbound online to inbounds that carried traffic

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

frontend/src/api/queryKeys.ts

@@ -22,6 +22,7 @@ export const keys = {
     all: () => ['clients', 'all'] as const,
     onlines: () => ['clients', 'onlines'] as const,
     onlinesByNode: () => ['clients', 'onlinesByNode'] as const,
+    activeInbounds: () => ['clients', 'activeInbounds'] as const,
     lastOnline: () => ['clients', 'lastOnline'] as const,
     groups: () => ['clients', 'groups'] as const,
   },