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/friends.go

@@ -29,6 +29,17 @@ const (
 // window; a one-time friend code from the addressee bypasses a decline.
 const friendRequestTTL = 30 * 24 * time.Hour
 
+// accountRef resolves accountID into a notify.AccountRef (the display name from the account
+// store, empty on a lookup failure), for enriching the friend_* live events so the client
+// updates its requests/friends state without a refetch (R4).
+func (svc *Service) accountRef(ctx context.Context, accountID uuid.UUID) notify.AccountRef {
+	ref := notify.AccountRef{AccountID: accountID.String()}
+	if acc, err := svc.accounts.GetByID(ctx, accountID); err == nil {
+		ref.DisplayName = acc.DisplayName
+	}
+	return ref
+}
+
 // SendFriendRequest records a pending friend request from requesterID to
 // addresseeID — the "befriend an opponent" path. It requires the two to share a
 // game (active or finished) and refuses a self-request, a request across a block or
@@ -91,7 +102,7 @@ func (svc *Service) SendFriendRequest(ctx context.Context, requesterID, addresse
 			if err := svc.store.refreshFriendRequest(ctx, requesterID, addresseeID, svc.now()); err != nil {
 				return err
 			}
-			svc.pub.Publish(notify.Notification(addresseeID, notify.NotifyFriendRequest))
+			svc.pub.Publish(notify.NotificationAccount(addresseeID, notify.NotifyFriendRequest, svc.accountRef(ctx, requesterID)))
 			return nil
 		}
 	}
@@ -101,7 +112,7 @@ func (svc *Service) SendFriendRequest(ctx context.Context, requesterID, addresse
 		}
 		return err
 	}
-	svc.pub.Publish(notify.Notification(addresseeID, notify.NotifyFriendRequest))
+	svc.pub.Publish(notify.NotificationAccount(addresseeID, notify.NotifyFriendRequest, svc.accountRef(ctx, requesterID)))
 	return nil
 }
 
@@ -128,9 +139,9 @@ func (svc *Service) RespondFriendRequest(ctx context.Context, addresseeID, reque
 	// this opponent re-derives its "add to friends" state (accepted -> friends, declined
 	// -> stays "request sent").
 	if accept {
-		svc.pub.Publish(notify.Notification(requesterID, notify.NotifyFriendAdded))
+		svc.pub.Publish(notify.NotificationAccount(requesterID, notify.NotifyFriendAdded, svc.accountRef(ctx, addresseeID)))
 	} else {
-		svc.pub.Publish(notify.Notification(requesterID, notify.NotifyFriendDeclined))
+		svc.pub.Publish(notify.NotificationAccount(requesterID, notify.NotifyFriendDeclined, svc.accountRef(ctx, addresseeID)))
 	}
 	return nil
 }