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/types.ts

@@ -36,6 +36,13 @@ export interface WsEventMap {
   /** Gateway emits { id, deleted_at } — NOT a bare string */
   message_deleted: { id: string; deleted_at?: number };
   message_analyzed: MessageRecord;
+  /**
+   * Streamed history frame — one MessageRecord per WS message (replaces the old
+   * 50-row batched `messages.list` fetch on the client). The view accumulates
+   * these into the SWR list as they arrive. `message_snapshot_end` signals done.
+   */
+  message_snapshot: MessageRecord;
+  message_snapshot_end: { sent: number; error?: boolean };
   attachment_created: unknown;
   attachment_uploaded: unknown;
   voice_recording_started: unknown;