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_social_test.go

@@ -153,7 +153,7 @@ func TestInvitationCreateRoundTrip(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/invitations" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"id":"i-1","inviter":{"account_id":"u-1","display_name":"Me"},"invitees":[{"account_id":"inv-1","display_name":"Friend","seat":1,"response":"pending"}],"variant":"english","turn_timeout_secs":86400,"hints_allowed":true,"hints_per_player":1,"dropout_tiles":"remove","status":"pending","expires_at_unix":42}`))
+		_, _ = w.Write([]byte(`{"id":"i-1","inviter":{"account_id":"u-1","display_name":"Me"},"invitees":[{"account_id":"inv-1","display_name":"Friend","seat":1,"response":"pending"}],"variant":"scrabble_en","turn_timeout_secs":86400,"hints_allowed":true,"hints_per_player":1,"dropout_tiles":"remove","status":"pending","expires_at_unix":42}`))
 	})
 	defer cleanup()
 
@@ -165,7 +165,7 @@ func TestInvitationCreateRoundTrip(t *testing.T) {
 	fb.CreateInvitationRequestStartInviteeIdsVector(b, 1)
 	b.PrependUOffsetT(inviteeID)
 	ids := b.EndVector(1)
-	variant := b.CreateString("english")
+	variant := b.CreateString("scrabble_en")
 	dropout := b.CreateString("remove")
 	fb.CreateInvitationRequestStart(b)
 	fb.CreateInvitationRequestAddInviteeIds(b, ids)
@@ -181,7 +181,7 @@ func TestInvitationCreateRoundTrip(t *testing.T) {
 		t.Fatalf("handler: %v", err)
 	}
 	inv := fb.GetRootAsInvitation(payload, 0)
-	if string(inv.Id()) != "i-1" || inv.InviteesLength() != 1 || string(inv.Variant()) != "english" {
+	if string(inv.Id()) != "i-1" || inv.InviteesLength() != 1 || string(inv.Variant()) != "scrabble_en" {
 		t.Fatalf("invitation decoded wrong: id=%q invitees=%d variant=%q", inv.Id(), inv.InviteesLength(), inv.Variant())
 	}
 	if iv := inv.Inviter(nil); iv == nil || string(iv.DisplayName()) != "Me" {