feat(#195): SSE realtime Cartes (stream + emit audience)

GET /api/v1/cards/stream (Bearer ou ?access_token=).
Events card.created|updated|deleted, response.added + heartbeat.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/modules/cards/index.ts

@@ -1,2 +1,3 @@
 export { CardsModule } from './cards.module';
 export { CardsService } from './cards.service';
+export { CardsRealtimeService } from './cards-realtime.service';