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/gcg_test.go

@@ -40,7 +40,7 @@ func TestWriteGCG(t *testing.T) {
 		"#character-encoding UTF-8",
 		"#player1 p1 Alice",
 		"#player2 p2 Bob",
-		"#lexicon english/v1",
+		"#lexicon scrabble_en/v1",
 		"#title game 00000000-0000-7000-8000-000000000001",
 		">p1: CATSER? 8H CAT +10 10",
 		">p2: AS?E I8 .s +2 2",

backend/internal/game/helpers_test.go

@@ -94,7 +94,7 @@ func TestGameCacheEviction(t *testing.T) {
 	cur := time.Unix(1_700_000_000, 0)
 	cache := newGameCache(time.Hour, func() time.Time { return cur })
 	id := uuid.New()
-	cache.put(id, nil, "english")
+	cache.put(id, nil, "scrabble_en")
 	if _, ok := cache.get(id); !ok {
 		t.Fatal("game must be resident after put")
 	}

backend/internal/game/metrics.go

@@ -16,7 +16,7 @@ import (
 const meterName = "scrabble/backend/game"
 
 // gameMetrics holds the game domain's operational instruments. Every game-scoped
-// measurement carries a "variant" attribute (english/russian/erudit). The
+// measurement carries a "variant" attribute (scrabble_en/scrabble_ru/erudit_ru). The
 // instruments default to no-ops (see defaultGameMetrics), so recording is always
 // safe; SetMetrics installs the real meter during startup wiring.
 type gameMetrics struct {

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)