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/game/Game.svelte

@@ -57,7 +57,7 @@
   let resignOpen = $state(false);
   let drag = $state<{ letter: string; blank: boolean; x: number; y: number; touch: boolean } | null>(null);
 
-  const variant = $derived(view?.game.variant ?? 'english');
+  const variant = $derived(view?.game.variant ?? 'scrabble_en');
   const board = $derived(replay(moves));
   const premium = $derived(premiumGrid(variant));
   const ctr = $derived(centre(variant));