perf(inbound): detach post-upsert work from buffered function

Removes the await around Promise.all([doAppStateSync, processMessage])
inside upsertMessage. createBufferedFunction (event-buffer.ts:819) only
schedules the buffer flush AFTER its work() callback resolves, so awaiting
post-upsert work pinned the messages.upsert emit inside the buffer for the
full duration of processMessage (signal cleanup, store writes, tcToken
maintenance, history mappings).

After this change the buffered function returns as soon as the synchronous
state-machine bookkeeping completes, the debounced flush releases
messages.upsert to the consumer, and processMessage / doAppStateSync run
in the background. Failures are caught and warn-logged so unhandled
rejections cannot crash the socket.

This is the last remaining sync hop on the inbound hot path that was still
adding latency on top of PR #390 (fire-and-forget LID mapping + tcToken
history sync).

src/Socket/chats.ts

@@ -1411,7 +1411,13 @@ export const makeChatsSocket = (config: SocketConfig) => {
 			}
 		}
 
-		await Promise.all([
+		// Fire-and-forget: post-upsert work (history app-state sync + processMessage)
+		// must NOT block the buffered function. Awaiting here keeps `messages.upsert`
+		// pinned in the event buffer (createBufferedFunction only schedules flush
+		// after work() resolves), delaying delivery to the consumer by the duration
+		// of processMessage. Detaching this work releases the emit on the next debounce
+		// tick while signal/store/tcToken side-effects continue in the background.
+		Promise.all([
 			(async () => {
 				if (shouldProcessHistoryMsg) {
 					await doAppStateSync()
@@ -1428,7 +1434,7 @@ export const makeChatsSocket = (config: SocketConfig) => {
 				options: config.options,
 				getMessage
 			})
-		])
+		]).catch(err => logger?.warn({ err }, 'background post-upsert work failed'))
 
 		// If the app state key arrives and we are waiting to sync, trigger the sync now.
 		if (msg.message?.protocolMessage?.appStateSyncKeyShare && syncState === SyncState.Syncing) {