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/game/metrics_test.go

@@ -35,11 +35,11 @@ func TestGameMetrics(t *testing.T) {
 	}
 
 	started := counterByAttr(t, rm, "games_started_total", "variant")
-	if started["english"] != 2 || started["russian_scrabble"] != 1 {
-		t.Errorf("games_started_total = %v, want english:2 russian_scrabble:1", started)
+	if started["scrabble_en"] != 2 || started["scrabble_ru"] != 1 {
+		t.Errorf("games_started_total = %v, want scrabble_en:2 scrabble_ru:1", started)
 	}
-	if abandoned := counterByAttr(t, rm, "games_abandoned_total", "variant"); abandoned["erudit"] != 1 {
-		t.Errorf("games_abandoned_total = %v, want erudit:1", abandoned)
+	if abandoned := counterByAttr(t, rm, "games_abandoned_total", "variant"); abandoned["erudit_ru"] != 1 {
+		t.Errorf("games_abandoned_total = %v, want erudit_ru:1", abandoned)
 	}
 	if c := histogramCount(t, rm, "game_replay_duration"); c != 1 {
 		t.Errorf("game_replay_duration observations = %d, want 1", c)