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/engine.go

@@ -38,11 +38,11 @@ const (
 func (v Variant) String() string {
 	switch v {
 	case VariantEnglish:
-		return "english"
+		return "scrabble_en"
 	case VariantRussianScrabble:
-		return "russian_scrabble"
+		return "scrabble_ru"
 	case VariantErudit:
-		return "erudit"
+		return "erudit_ru"
 	}
 	return "unknown"
 }