R4: push enrichment — events carry a state delta, kill the last poll

Enrich the in-app live stream into a delta channel so the UI renders a move from the event without a follow-up game.state, and make the matchmaking poll a stream-down fallback.

- pkg/fbs: trailing fields on opponent_moved (move+game+bag_len), your_turn (move_count), match_found (state), game_over (game), notify (account/invitation/state), MoveResult (rack+bag_len); regenerate Go + TS.
- backend: notify owns the FB encoding (encode.go + payload.go input structs); game/lobby/social map their domain types in. emitMove builds the move delta; game.Service.InitialState feeds match_found/game_started the recipient's initial StateView; friends/invitations notify carry their account/invitation. The move-commit response (submit_play/pass/exchange/resign) returns the actor's refilled rack + bag size.
- gateway: MoveResult transcode carries rack+bag_len.
- ui: pure lib/gamedelta.ts reducer advances the per-game cache keyed on move_count (idempotent + gap-safe); app.svelte seeds the cache on match_found/game_started; Game.svelte applies the delta (commit/pass/exchange/resign drop their load()); NewGame polls only while app.streamAlive is false.
- docs: ARCHITECTURE §10, FUNCTIONAL(+ru), backend/gateway/ui READMEs; PRERELEASE R4 marked done + Refinements.

backend/internal/social/friendcodes.go

@@ -82,7 +82,7 @@ func (svc *Service) RedeemFriendCode(ctx context.Context, redeemerID uuid.UUID,
 	if err := svc.store.redeemFriendCode(ctx, codeID, issuerID, redeemerID, svc.now()); err != nil {
 		return uuid.UUID{}, err
 	}
-	svc.pub.Publish(notify.Notification(issuerID, notify.NotifyFriendAdded))
+	svc.pub.Publish(notify.NotificationAccount(issuerID, notify.NotifyFriendAdded, svc.accountRef(ctx, redeemerID)))
 	return issuerID, nil
 }