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.

ui/src/lib/lobbysort.test.ts

@@ -15,7 +15,7 @@ const seat = (s: number, accountId: string): Seat => ({
 function game(id: string, status: GameView['status'], toMove: number, lastActivityUnix: number): GameView {
   return {
     id,
-    variant: 'english',
+    variant: 'scrabble_en',
     dictVersion: 'v1',
     status,
     players: 2,