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/e2e/social.spec.ts

@@ -131,7 +131,7 @@ test('play with friends: a game type is required to send an invitation', async (
   await page.getByRole('checkbox').first().check(); // pick a friend
   await expect(send).toBeDisabled(); // still no game type
 
-  await page.locator('.field select').first().selectOption('english');
+  await page.locator('.field select').first().selectOption('scrabble_en');
   await expect(send).toBeEnabled();
 
   await send.click(); // the mock creates it and returns to the lobby