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/engine/registry_test.go

@@ -60,7 +60,7 @@ func TestRegistryValidatesKnownWords(t *testing.T) {
 func TestRegistryUnknownLookups(t *testing.T) {
 	reg, err := Open(testDictDir(), testVersion, VariantEnglish)
 	if err != nil {
-		t.Fatalf("open english-only registry: %v", err)
+		t.Fatalf("open scrabble_en-only registry: %v", err)
 	}
 	defer reg.Close()