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/domain_test.go

@@ -168,10 +168,10 @@ func TestRegistryLookup(t *testing.T) {
 		word    string
 		want    bool
 	}{
-		{"english hit", VariantEnglish, "cat", true},
-		{"english miss", VariantEnglish, "zzzz", false},
-		{"russian hit", VariantRussianScrabble, "кот", true},
-		{"erudit hit", VariantErudit, "кот", true},
+		{"scrabble_en hit", VariantEnglish, "cat", true},
+		{"scrabble_en miss", VariantEnglish, "zzzz", false},
+		{"scrabble_ru hit", VariantRussianScrabble, "кот", true},
+		{"erudit_ru hit", VariantErudit, "кот", true},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {