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/model.ts

@@ -3,7 +3,7 @@
 // FlatBuffers) and the mock transport speak this model, so the UI never touches
 // generated wire code directly.
 
-export type Variant = 'english' | 'russian_scrabble' | 'erudit';
+export type Variant = 'scrabble_en' | 'scrabble_ru' | 'erudit_ru';
 
 /** Backend game status strings. */
 export type GameStatus = 'active' | 'finished' | string;