R6(c): drop dead opponent_moved scalars (seat/action/score/total)

These pre-R4 summary scalars on OpponentMovedEvent were redundant with the
move/game delta and read by nobody — the UI codec and mock take only
move/game/bag_len, and the gateway forwards the push payload verbatim. Removed
from scrabble.fbs, the notify emit (notify/events.go) and the round-trip test;
regenerated the FB Go + TS bindings. No prod data, so the wire-slot renumber is
free and there is no DB change.

backend/internal/notify/notify_test.go

@@ -109,10 +109,8 @@ func TestOpponentMovedPayloadRoundTrips(t *testing.T) {
 		t.Fatalf("kind = %q", in.Kind)
 	}
 	ev := fb.GetRootAsOpponentMovedEvent(in.Payload, 0)
-	// The summary scalars repeat the move.
-	if string(ev.GameId()) != gid.String() || ev.Seat() != 1 || string(ev.Action()) != "play" || ev.Score() != 24 || ev.Total() != 130 {
-		t.Fatalf("scalars wrong: game=%q seat=%d action=%q score=%d total=%d",
-			ev.GameId(), ev.Seat(), ev.Action(), ev.Score(), ev.Total())
+	if string(ev.GameId()) != gid.String() {
+		t.Fatalf("game id = %q", ev.GameId())
 	}
 	// The delta: the move, the post-move summary and the bag size.
 	if ev.BagLen() != 42 {