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/mock/client.ts

@@ -61,9 +61,9 @@ function emptyLinked(): LinkResult {
 }
 
 const POOL: Record<Variant, string> = {
-  english: 'AAAAEEEEIIIOONNRRTTLLSSUDGBCMPFHVWYK',
-  russian_scrabble: 'ОООААЕЕИИННТТСРРВЛКМДПУЯЫЬГЗБ',
-  erudit: 'ОООААЕЕИИННТТСРРВЛКМДПУЯЫЬГЗБ',
+  scrabble_en: 'AAAAEEEEIIIOONNRRTTLLSSUDGBCMPFHVWYK',
+  scrabble_ru: 'ОООААЕЕИИННТТСРРВЛКМДПУЯЫЬГЗБ',
+  erudit_ru: 'ОООААЕЕИИННТТСРРВЛКМДПУЯЫЬГЗБ',
 };
 
 function draw(variant: Variant, n: number): string[] {