feat(messages): stream history one-message-per-WS-frame instead of 50-row batch

- backend: add streamMany generator (paginated, yields one record at a time)
  + messagesService.streamMessages + WS 'stream_messages' handler emitting
  'message_snapshot' per message, 'message_snapshot_end' with nextCursor
- frontend: useMessagesStream hook accumulates snapshots into SWR list,
  SSR getMessages seeds first paint, WsHook gains sendText
- add stream-many.test.ts locking the one-at-a-time + cursor contract

services/frontend/src/hooks/index.ts

@@ -25,6 +25,7 @@ export {
   useMessageSearch,
   useMessages,
   useMessagesHasMore,
+  useMessagesStream,
   useMessagesWsSync,
   useReview,
   useTextChannels,