R1: schema & naming reset — squash migrations, rename variants

Squash the 12 goose migrations into one 00001_baseline.sql (there is no prod
data; verified schema-identical to the chain via a pg_dump diff + the green
integration suite) and rename the game-variant labels
english/russian_scrabble/erudit -> scrabble_en/scrabble_ru/erudit_ru across the
backend, the FlatBuffers wire values and the UI.

dawg filenames and the Go enum identifiers are unchanged; the i18n display keys
are kept. Adds PRERELEASE.md (the R1-R7 pre-release tracker), linked from
CLAUDE.md. Contour DB wipe and the scrabble-dictionary tidy are follow-ups.

backend/internal/inttest/game_test.go

@@ -477,7 +477,7 @@ func TestGameVariant(t *testing.T) {
 		t.Fatalf("create: %v", err)
 	}
 	if v, err := svc.GameVariant(ctx, g.ID); err != nil || v != engine.VariantEnglish {
-		t.Fatalf("GameVariant = %v, %v; want english, nil", v, err)
+		t.Fatalf("GameVariant = %v, %v; want scrabble_en, nil", v, err)
 	}
 	if _, err := svc.GameVariant(ctx, uuid.New()); !errors.Is(err, game.ErrNotFound) {
 		t.Errorf("GameVariant(unknown) = %v, want ErrNotFound", err)