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/schemas/client.ts

@@ -117,6 +117,11 @@ export const OnlineByNodeSchema = z
   .nullable()
   .transform((v) => v ?? {});
 
+export const ActiveInboundsByNodeSchema = z
+  .record(z.string(), nullableStringArray)
+  .nullable()
+  .transform((v) => v ?? {});
+
 export const GroupSummarySchema = z.object({
   name: z.string(),
   clientCount: z.number(),