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/postgres/migrations/00014_single_bot_variant_preferences.sql

@@ -0,0 +1,45 @@
+-- +goose Up
+-- Single-bot collapse + per-user variant preferences.
+--
+-- With one Telegram bot there is no longer a "service language" (which bot a player
+-- signed in through), and variant availability is no longer gated by the bot's
+-- supported languages but by an explicit per-account preference. variant_preferences
+-- is the set of game variants (engine.Variant stable labels) a player is willing to be
+-- matched into: it gates the New Game picker and the matchmaker, while an invited friend
+-- may still accept any variant. New accounts default to Erudit only (a DB-level default,
+-- so no application seed is needed); the set may never be empty and must be a subset of
+-- the three known variants. feedback_messages.channel_lang (the bot a message arrived
+-- through) likewise loses meaning under one bot and is dropped; the sender's interface
+-- language (lang) is kept.
+SET search_path = backend, pg_catalog;
+
+ALTER TABLE accounts
+    ADD COLUMN variant_preferences text[] NOT NULL DEFAULT ARRAY['erudit_ru']::text[],
+    ADD CONSTRAINT accounts_variant_preferences_nonempty_chk
+        CHECK (cardinality(variant_preferences) >= 1),
+    ADD CONSTRAINT accounts_variant_preferences_subset_chk
+        CHECK (variant_preferences <@ ARRAY['scrabble_en', 'scrabble_ru', 'erudit_ru']::text[]);
+
+ALTER TABLE accounts DROP COLUMN service_language;
+
+ALTER TABLE feedback_messages DROP COLUMN channel_lang;
+
+-- The auto-match pairing query filters open games by (variant, multiple_words_per_turn)
+-- and takes the oldest by created_at; games_open_idx (open_deadline_at) does not serve it.
+-- A partial index over open games turns the scan-and-sort into a single index seek.
+CREATE INDEX games_open_match_idx ON games (variant, multiple_words_per_turn, created_at)
+    WHERE status = 'open';
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+
+DROP INDEX games_open_match_idx;
+
+ALTER TABLE feedback_messages ADD COLUMN channel_lang text;
+
+ALTER TABLE accounts ADD COLUMN service_language text CHECK (service_language IN ('en', 'ru'));
+
+ALTER TABLE accounts
+    DROP CONSTRAINT accounts_variant_preferences_subset_chk,
+    DROP CONSTRAINT accounts_variant_preferences_nonempty_chk,
+    DROP COLUMN variant_preferences;