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/CheckScreen.svelte

@@ -12,7 +12,7 @@
   // complaint, off the board so the soft keyboard never relayouts the play area.
   let { id }: { id: string } = $props();
 
-  let variant = $state<Variant>('english');
+  let variant = $state<Variant>('scrabble_en');
   let word = $state('');
   let result = $state<{ word: string; legal: boolean } | null>(null);
   let cooling = $state(false);