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.

pkg/fbs/scrabblefb/UpdateProfileRequest.go

@@ -117,8 +117,25 @@ func (rcv *UpdateProfileRequest) MutateNotificationsInAppOnly(n bool) bool {
 	return rcv._tab.MutateBoolSlot(18, n)
 }
 
+func (rcv *UpdateProfileRequest) VariantPreferences(j int) []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(20))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+	}
+	return nil
+}
+
+func (rcv *UpdateProfileRequest) VariantPreferencesLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(20))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
 func UpdateProfileRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(8)
+	builder.StartObject(9)
 }
 func UpdateProfileRequestAddDisplayName(builder *flatbuffers.Builder, displayName flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(displayName), 0)
@@ -144,6 +161,12 @@ func UpdateProfileRequestAddBlockFriendRequests(builder *flatbuffers.Builder, bl
 func UpdateProfileRequestAddNotificationsInAppOnly(builder *flatbuffers.Builder, notificationsInAppOnly bool) {
 	builder.PrependBoolSlot(7, notificationsInAppOnly, true)
 }
+func UpdateProfileRequestAddVariantPreferences(builder *flatbuffers.Builder, variantPreferences flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(8, flatbuffers.UOffsetT(variantPreferences), 0)
+}
+func UpdateProfileRequestStartVariantPreferencesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
 func UpdateProfileRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }