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/account/profile.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math/rand/v2"
 	"regexp"
+	"slices"
 	"strings"
 	"time"
 	"unicode"
@@ -76,6 +77,18 @@ var knownVariants = map[string]bool{"erudit_ru": true, "scrabble_ru": true, "scr
 // in (Erudit, Russian Scrabble, English), independent of the client's order.
 var canonicalVariantOrder = []string{"erudit_ru", "scrabble_ru", "scrabble_en"}
 
+// DefaultVariantPreferences is the variant set a newly registered account starts with:
+// both Russian-alphabet games. It mirrors the accounts.variant_preferences column
+// default, which is what actually seeds a created row — the constant is here so the
+// promotion of a guest (ClearGuest) and the tests name the same set as the schema.
+var DefaultVariantPreferences = []string{"erudit_ru", "scrabble_ru"}
+
+// GuestVariantPreferences is the variant set an ephemeral guest starts with: Эрудит
+// alone. A guest is deliberately narrower than a registered player — the New Game
+// screen offers the single variant and invites the guest to register for the rest —
+// so ProvisionGuest writes it explicitly instead of taking the column default.
+var GuestVariantPreferences = []string{"erudit_ru"}
+
 // validateVariantPreferences cleans a profile's variant-preference set: it drops
 // duplicates, rejects an unknown label or an empty set (ErrInvalidProfile) and
 // returns the preferences in canonicalVariantOrder so the stored value is
@@ -129,6 +142,33 @@ func SeedVariantsFromStartParam(startParam string) []string {
 	return prefs
 }
 
+// MergeVariantSeed resolves the variant-preference set to store for an account that
+// arrived on a promo deep link. The seed decoded from the link is *added* to current
+// (the set the account already carries, normally the column default) rather than
+// replacing it, so a campaign can only ever widen a player's choice. English Scrabble
+// is the one variant the language gates: it is dropped from seed when languageCode is
+// Russian, because a Russian-speaking arrival is served by the two Russian-alphabet
+// games and the English one would only clutter New Game. It returns nil when there is
+// nothing to write — an empty or invalid seed, or a merge that leaves current as it is
+// — so the caller can skip the update.
+func MergeVariantSeed(current, seed []string, languageCode string) []string {
+	if len(seed) == 0 {
+		return nil
+	}
+	extra := seed
+	if supportedLanguage(languageCode) == "ru" {
+		extra = slices.DeleteFunc(slices.Clone(seed), func(v string) bool { return v == "scrabble_en" })
+	}
+	merged, err := validateVariantPreferences(append(slices.Clone(current), extra...))
+	if err != nil {
+		return nil
+	}
+	if base, err := validateVariantPreferences(current); err == nil && slices.Equal(base, merged) {
+		return nil
+	}
+	return merged
+}
+
 // SetVariantPreferences overwrites only the variant-preference set of the account,
 // cleaning it to a deduplicated, canonically ordered subset of the known variants
 // (rejecting an empty or unknown set with ErrInvalidProfile) and bumping updated_at; it