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/variants.ts

@@ -17,9 +17,9 @@ export interface VariantOption {
 // two never collide whatever the UI language); Erudit is localized "Erudite"/"Эрудит"
 // (Stage 17).
 export const ALL_VARIANTS: VariantOption[] = [
-  { id: 'english', label: 'new.english' },
-  { id: 'russian_scrabble', label: 'new.russian' },
-  { id: 'erudit', label: 'new.erudit' },
+  { id: 'scrabble_en', label: 'new.english' },
+  { id: 'scrabble_ru', label: 'new.russian' },
+  { id: 'erudit_ru', label: 'new.erudit' },
 ];
 
 // variantNameKey returns the i18n key for a variant's display name (used by the in-game
@@ -31,22 +31,22 @@ export function variantNameKey(v: Variant): MessageKey {
 // VARIANT_RULES is the i18n key for each variant's one-line rules summary on the New Game
 // buttons (bag size, the ё rule, bonus differences), sourced from the engine rulesets.
 export const VARIANT_RULES: Record<Variant, MessageKey> = {
-  english: 'new.rulesEnglish',
-  russian_scrabble: 'new.rulesRussian',
-  erudit: 'new.rulesErudit',
+  scrabble_en: 'new.rulesEnglish',
+  scrabble_ru: 'new.rulesRussian',
+  erudit_ru: 'new.rulesErudit',
 };
 
 // VARIANT_FLAG is the flag shown on a variant button: an emoji for the Scrabble variants;
 // Erudit uses the bundled USSR flag SVG (public/flag-ussr.svg), so its entry is empty.
 export const VARIANT_FLAG: Record<Variant, string> = {
-  english: '🇺🇸',
-  russian_scrabble: '🇷🇺',
-  erudit: '',
+  scrabble_en: '🇺🇸',
+  scrabble_ru: '🇷🇺',
+  erudit_ru: '',
 };
 
 // VARIANT_LANGUAGE maps each variant to its game language. en -> English;
 // ru -> Russian + Эрудит.
-export const VARIANT_LANGUAGE: Record<Variant, 'en' | 'ru'> = { english: 'en', russian_scrabble: 'ru', erudit: 'ru' };
+export const VARIANT_LANGUAGE: Record<Variant, 'en' | 'ru'> = { scrabble_en: 'en', scrabble_ru: 'ru', erudit_ru: 'ru' };
 
 // availableVariants gates ALL_VARIANTS by the session's supported languages. An empty
 // or absent set is ungated (a web/legacy session without a declared set), returning