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/data.ts

@@ -57,7 +57,7 @@ export function mockInvitations(): Invitation[] {
       id: 'inv1',
       inviter: { accountId: 'kaya', displayName: 'Kaya' },
       invitees: [{ accountId: ME, displayName: 'You', seat: 1, response: 'pending' }],
-      variant: 'english',
+      variant: 'scrabble_en',
       turnTimeoutSecs: 86400,
       hintsAllowed: true,
       hintsPerPlayer: 1,
@@ -105,7 +105,7 @@ export interface MockGame {
   chat: ChatMessage[];
 }
 
-// --- active game G1: english, You (seat 0) vs Ann (seat 1), your turn ---
+// --- active game G1: scrabble_en, You (seat 0) vs Ann (seat 1), your turn ---
 
 const G1_MOVES: MoveRecord[] = [
   play(0, 'H', [
@@ -135,7 +135,7 @@ function activeGame(): MockGame {
   return {
     view: {
       id: 'g1',
-      variant: 'english',
+      variant: 'scrabble_en',
       dictVersion: 'v1',
       status: 'active',
       players: 2,
@@ -169,7 +169,7 @@ function finishedG2(): MockGame {
   return {
     view: {
       id: 'g2',
-      variant: 'english',
+      variant: 'scrabble_en',
       dictVersion: 'v1',
       status: 'finished',
       players: 2,
@@ -204,7 +204,7 @@ function finishedG3(): MockGame {
   return {
     view: {
       id: 'g3',
-      variant: 'russian_scrabble',
+      variant: 'scrabble_ru',
       dictVersion: 'v1',
       status: 'finished',
       players: 2,