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/lib/ws-hook.ts

@@ -5,4 +5,5 @@ export type WsHook = {
     eventType: E,
     handler: (data: unknown) => void,
   ) => () => void;
+  sendText: (text: string) => void;
 };