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",