feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

backend/internal/account/profile.go

@@ -14,6 +14,7 @@ import (
 	"github.com/go-jet/jet/v2/postgres"
 	"github.com/go-jet/jet/v2/qrm"
 	"github.com/google/uuid"
+	"github.com/lib/pq"
 
 	"scrabble/backend/internal/postgres/jet/backend/model"
 	"scrabble/backend/internal/postgres/jet/backend/table"
@@ -58,6 +59,46 @@ type ProfileUpdate struct {
 	BlockChat              bool
 	BlockFriendRequests    bool
 	NotificationsInAppOnly bool
+	// VariantPreferences is the set of game variants the player allows themselves to
+	// be matched into (engine.Variant stable labels). UpdateProfile cleans it to a
+	// deduplicated, canonically ordered subset of the known variants and rejects an
+	// empty set.
+	VariantPreferences []string
+}
+
+// knownVariants is the closed set of game-variant labels (engine.Variant stable
+// labels) a profile's variant preferences may contain. It lives here so the store
+// does not depend on the engine package; the server handler additionally validates
+// against engine.ParseVariant, and a DB check enforces the same subset.
+var knownVariants = map[string]bool{"erudit_ru": true, "scrabble_ru": true, "scrabble_en": true}
+
+// canonicalVariantOrder is the deterministic order variant preferences are stored
+// in (Erudit, Russian Scrabble, English), independent of the client's order.
+var canonicalVariantOrder = []string{"erudit_ru", "scrabble_ru", "scrabble_en"}
+
+// 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
+// deterministic regardless of the order the client sent.
+func validateVariantPreferences(prefs []string) ([]string, error) {
+	seen := make(map[string]bool, len(prefs))
+	for _, p := range prefs {
+		p = strings.TrimSpace(p)
+		if !knownVariants[p] {
+			return nil, fmt.Errorf("%w: variant preference %q", ErrInvalidProfile, p)
+		}
+		seen[p] = true
+	}
+	if len(seen) == 0 {
+		return nil, fmt.Errorf("%w: variant preferences must not be empty", ErrInvalidProfile)
+	}
+	out := make([]string, 0, len(seen))
+	for _, v := range canonicalVariantOrder {
+		if seen[v] {
+			out = append(out, v)
+		}
+	}
+	return out, nil
 }
 
 // UpdateProfile validates and overwrites the editable fields of the account, then
@@ -79,17 +120,26 @@ func (s *Store) UpdateProfile(ctx context.Context, id uuid.UUID, p ProfileUpdate
 	if err := validateAwayWindow(p.AwayStart, p.AwayEnd); err != nil {
 		return Account{}, err
 	}
+	prefs, err := validateVariantPreferences(p.VariantPreferences)
+	if err != nil {
+		return Account{}, err
+	}
 
 	stmt := table.Accounts.UPDATE(
 		table.Accounts.DisplayName, table.Accounts.PreferredLanguage, table.Accounts.TimeZone,
 		table.Accounts.AwayStart, table.Accounts.AwayEnd,
 		table.Accounts.BlockChat, table.Accounts.BlockFriendRequests,
-		table.Accounts.NotificationsInAppOnly, table.Accounts.UpdatedAt,
+		table.Accounts.NotificationsInAppOnly, table.Accounts.VariantPreferences,
+		table.Accounts.UpdatedAt,
 	).SET(
 		postgres.String(name), postgres.String(lang), postgres.String(tz),
 		postgres.TimeT(p.AwayStart), postgres.TimeT(p.AwayEnd),
 		postgres.Bool(p.BlockChat), postgres.Bool(p.BlockFriendRequests),
-		postgres.Bool(p.NotificationsInAppOnly), postgres.TimestampzT(time.Now().UTC()),
+		postgres.Bool(p.NotificationsInAppOnly),
+		// prefs are validated against the closed knownVariants set; bind as a text[]
+		// parameter (lib/pq encodes the array, the cast pins the column type).
+		postgres.Raw("#variant_prefs::text[]", map[string]interface{}{"#variant_prefs": pq.StringArray(prefs)}),
+		postgres.TimestampzT(time.Now().UTC()),
 	).WHERE(table.Accounts.AccountID.EQ(postgres.UUID(id))).
 		RETURNING(table.Accounts.AllColumns)