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/robot_test.go

@@ -90,7 +90,7 @@ func TestRobotPoolProvisionsRobotAccounts(t *testing.T) {
 		t.Errorf("picked account %s is not a robot identity", id)
 	}
 	if ru, err := r.Pick(engine.VariantRussianScrabble); err != nil || !isRobotAccount(t, ru) {
-		t.Errorf("russian pick = (%s, %v), want a robot account", ru, err)
+		t.Errorf("scrabble_ru pick = (%s, %v), want a robot account", ru, err)
 	}
 	acc, err := account.NewStore(testDB).GetByID(ctx, id)
 	if err != nil {