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/reload_test.go

@@ -45,13 +45,13 @@ func TestLoadAvailableLoadsPresentSkipsAbsent(t *testing.T) {
 		t.Fatalf("load available: %v", err)
 	}
 	if len(loaded) != 1 || loaded[0] != VariantEnglish {
-		t.Fatalf("loaded = %v, want [english]", loaded)
+		t.Fatalf("loaded = %v, want [scrabble_en]", loaded)
 	}
 	if _, err := reg.Solver(VariantEnglish, "v2"); err != nil {
-		t.Errorf("english v2 solver: %v", err)
+		t.Errorf("scrabble_en v2 solver: %v", err)
 	}
 	if _, err := reg.Solver(VariantRussianScrabble, "v2"); !errors.Is(err, ErrUnknownVariant) {
-		t.Errorf("russian v2 should be absent: got %v", err)
+		t.Errorf("scrabble_ru v2 should be absent: got %v", err)
 	}
 }
 
@@ -77,17 +77,17 @@ func TestOpenWithVersionsScansSubdirs(t *testing.T) {
 		}
 	}
 	if got := reg.Versions(VariantEnglish); len(got) != 2 {
-		t.Errorf("english versions = %v, want two", got)
+		t.Errorf("scrabble_en versions = %v, want two", got)
 	}
 	latest, _, err := reg.Latest(VariantEnglish)
 	if err != nil {
-		t.Fatalf("latest english: %v", err)
+		t.Fatalf("latest scrabble_en: %v", err)
 	}
 	if latest != "v2" {
-		t.Errorf("latest english = %q, want v2", latest)
+		t.Errorf("latest scrabble_en = %q, want v2", latest)
 	}
 	if got := reg.Versions(VariantRussianScrabble); len(got) != 1 {
-		t.Errorf("russian versions = %v, want one (no v2 file)", got)
+		t.Errorf("scrabble_ru versions = %v, want one (no v2 file)", got)
 	}
 }