feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

backend/internal/postgres/migrations/00018_default_variant_preferences.sql

@@ -0,0 +1,16 @@
+-- Registered players start with both Russian-alphabet games enabled: Эрудит and Russian Scrabble.
+-- Only the column default moves — existing accounts keep the set they already carry, so a player
+-- who deliberately settled on one variant is not overruled. Guests are the exception and stay on
+-- Эрудит alone; account.ProvisionGuest therefore writes the column explicitly rather than relying
+-- on this default, and account.ClearGuest widens a promoted guest to the default set. A default
+-- change rewrites no rows (no contour wipe); an image rollback ignores it — the previous code
+-- reads a two-element set unchanged.
+-- +goose Up
+
+ALTER TABLE backend.accounts
+    ALTER COLUMN variant_preferences SET DEFAULT ARRAY['erudit_ru', 'scrabble_ru']::text[];
+
+-- +goose Down
+
+ALTER TABLE backend.accounts
+    ALTER COLUMN variant_preferences SET DEFAULT ARRAY['erudit_ru']::text[];