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/analytics_test.go

@@ -33,7 +33,7 @@ func TestMoveDurationAnalytics(t *testing.T) {
 	t0 := time.Date(2026, 1, 1, 12, 0, 0, 0, time.UTC)
 	if _, err := testDB.ExecContext(ctx,
 		`INSERT INTO backend.games (game_id, variant, dict_version, seed, players, turn_timeout_secs, created_at)
-		 VALUES ($1,'english','v1',1,2,86400,$2)`, gid, t0); err != nil {
+		 VALUES ($1,'scrabble_en','v1',1,2,86400,$2)`, gid, t0); err != nil {
 		t.Fatalf("insert game: %v", err)
 	}
 	if _, err := testDB.ExecContext(ctx,