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.

gateway/internal/transcode/transcode_alphabet_test.go

@@ -21,7 +21,7 @@ func TestGameStateIncludesAlphabet(t *testing.T) {
 		if got := r.URL.Query().Get("include_alphabet"); got != "true" {
 			t.Errorf("include_alphabet query = %q, want true", got)
 		}
-		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[0,255],"bag_len":50,"hints_remaining":0,"alphabet":[{"index":0,"letter":"a","value":1},{"index":1,"letter":"b","value":3}]}`))
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"scrabble_en","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[0,255],"bag_len":50,"hints_remaining":0,"alphabet":[{"index":0,"letter":"a","value":1},{"index":1,"letter":"b","value":3}]}`))
 	})
 	defer cleanup()
 
@@ -60,7 +60,7 @@ func TestGameStateOmitsAlphabetByDefault(t *testing.T) {
 		if r.URL.Query().Get("include_alphabet") == "true" {
 			t.Error("include_alphabet should be unset")
 		}
-		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[2,0,19],"bag_len":50,"hints_remaining":0}`))
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"scrabble_en","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[2,0,19],"bag_len":50,"hints_remaining":0}`))
 	})
 	defer cleanup()