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/server/handlers_user.go

@@ -2,8 +2,10 @@ package server
 
 import (
 	"net/http"
+	"slices"
 
 	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
 
 	"scrabble/backend/internal/engine"
 )
@@ -134,6 +136,24 @@ func (s *Server) handleGameState(c *gin.Context) {
 	c.JSON(http.StatusOK, dto)
 }
 
+// ensureVariantAllowed reports whether the caller's profile permits creating a game
+// of variant — it must be one of their VariantPreferences. It aborts the request
+// with 400 and returns false otherwise. Only the player who creates a game (quick
+// match, vs-AI or a friend invitation) is gated this way; an invited friend may
+// accept an invitation in any variant.
+func (s *Server) ensureVariantAllowed(c *gin.Context, uid uuid.UUID, variant string) bool {
+	acc, err := s.accounts.GetByID(c.Request.Context(), uid)
+	if err != nil {
+		s.abortErr(c, err)
+		return false
+	}
+	if !slices.Contains(acc.VariantPreferences, variant) {
+		abortBadRequest(c, "variant is not in your preferences")
+		return false
+	}
+	return true
+}
+
 // enqueueRequest enters per-variant auto-match under a per-turn word rule. VsAI true
 // starts an honest-AI game against a robot instead of the open/wait matchmaking path.
 type enqueueRequest struct {
@@ -162,6 +182,9 @@ func (s *Server) handleEnqueue(c *gin.Context) {
 		abortBadRequest(c, "unknown variant")
 		return
 	}
+	if !s.ensureVariantAllowed(c, uid, variant.String()) {
+		return
+	}
 	if !s.ensureUnderGameLimit(c, uid) {
 		return
 	}