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/proto/push/v1/push.pb.go

@@ -79,16 +79,11 @@ func (x *SubscribeRequest) GetGatewayId() string {
 // FlatBuffers-encoded body for that kind. event_id is a correlation id the
 // gateway carries through to the client envelope.
 type Event struct {
-	state   protoimpl.MessageState `protogen:"open.v1"`
-	UserId  string                 `protobuf:"bytes,1,opt,name=user_id,json=userId,proto3" json:"user_id,omitempty"`
-	Kind    string                 `protobuf:"bytes,2,opt,name=kind,proto3" json:"kind,omitempty"`
-	Payload []byte                 `protobuf:"bytes,3,opt,name=payload,proto3" json:"payload,omitempty"`
-	EventId string                 `protobuf:"bytes,4,opt,name=event_id,json=eventId,proto3" json:"event_id,omitempty"`
-	// language routes an out-of-app push to a specific per-language bot: for a game
-	// event it carries the game's language ("en"/"ru") so the notification comes from the game's
-	// bot rather than the recipient's last-login bot. Empty falls back to the recipient's service
-	// language at the gateway.
-	Language      string `protobuf:"bytes,5,opt,name=language,proto3" json:"language,omitempty"`
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	UserId        string                 `protobuf:"bytes,1,opt,name=user_id,json=userId,proto3" json:"user_id,omitempty"`
+	Kind          string                 `protobuf:"bytes,2,opt,name=kind,proto3" json:"kind,omitempty"`
+	Payload       []byte                 `protobuf:"bytes,3,opt,name=payload,proto3" json:"payload,omitempty"`
+	EventId       string                 `protobuf:"bytes,4,opt,name=event_id,json=eventId,proto3" json:"event_id,omitempty"`
 	unknownFields protoimpl.UnknownFields
 	sizeCache     protoimpl.SizeCache
 }
@@ -151,13 +146,6 @@ func (x *Event) GetEventId() string {
 	return ""
 }
 
-func (x *Event) GetLanguage() string {
-	if x != nil {
-		return x.Language
-	}
-	return ""
-}
-
 var File_push_v1_push_proto protoreflect.FileDescriptor
 
 const file_push_v1_push_proto_rawDesc = "" +
@@ -165,13 +153,12 @@ const file_push_v1_push_proto_rawDesc = "" +
 	"\x12push/v1/push.proto\x12\x10scrabble.push.v1\"1\n" +
 	"\x10SubscribeRequest\x12\x1d\n" +
 	"\n" +
-	"gateway_id\x18\x01 \x01(\tR\tgatewayId\"\x85\x01\n" +
+	"gateway_id\x18\x01 \x01(\tR\tgatewayId\"i\n" +
 	"\x05Event\x12\x17\n" +
 	"\auser_id\x18\x01 \x01(\tR\x06userId\x12\x12\n" +
 	"\x04kind\x18\x02 \x01(\tR\x04kind\x12\x18\n" +
 	"\apayload\x18\x03 \x01(\fR\apayload\x12\x19\n" +
-	"\bevent_id\x18\x04 \x01(\tR\aeventId\x12\x1a\n" +
-	"\blanguage\x18\x05 \x01(\tR\blanguage2R\n" +
+	"\bevent_id\x18\x04 \x01(\tR\aeventId2R\n" +
 	"\x04Push\x12J\n" +
 	"\tSubscribe\x12\".scrabble.push.v1.SubscribeRequest\x1a\x17.scrabble.push.v1.Event0\x01B#Z!scrabble/pkg/proto/push/v1;pushv1b\x06proto3"