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.

PRERELEASE.md

@@ -37,6 +37,7 @@ the edge before prod. Each phase maps back to the owner's raw pre-release TODO l
 | CR | In-game chat read receipts: per-message `unread_seats` bitmask (migration `00008`); a per-viewer unread **dot** in the lobby + game header (a nudge counts and clears when its recipient moves); reading = opening the move history (the 💬 fade-blinks twice) or the chat, acked (`chat.read`) only when unread; `chat_read_duration` + `chat_unread_messages` metrics + tracing + the **Scrabble — Messages** Grafana dashboard (follow-up PR); a message to a disguised robot opponent is born read; admin unread-only filter / read column / per-seat read card | owner ad-hoc | **done** |
 | BX | Asymmetric per-user block + in-game controls: a block now silently suppresses everything **from** the blocked user (chat, nudge, friend requests, invitations are kept but never delivered/surfaced, born-read) while they notice nothing, **without** deleting the friendship (unblock restores it); auto-match excludes a block-related pair (either direction); in-game opponent card gains a ✖️ **block** control (mirroring 🤝, red "Block?" confirm, mutual-hide, struck name + hidden chat composer when blocked); optimistic apply + `user_blocked`/`user_unblocked` event confirm + rollback; admin user card gains **blocks / blocked-by / friends** cross-linked lists. Blocking a disguised-robot opponent is recorded per-game in a separate **`robot_blocks`** table (migration `00011`), keyed on game+seat with the seen name — never the shared robot account — so the matchmaker keeps giving robots; it shows in the blocked list and re-marks the in-game card | owner ad-hoc | **done** |
 | FM | First-move tile draw (official rules): each seated player draws a tile, the one closest to "A" leads (a blank beats every letter), ties re-drawing until a single leader; **honest per-draw `crypto/rand` entropy**, not the bag seed, so the **record** (`game_setup_draws`, migration `00013`) — not a seed — is the only account of the outcome, kept for future **tournaments** (designed as a discrete per-tile "player N draws" step). Friend/AI draws at create; **auto-match draws at *open*** against a synthetic `uuid.Nil` opponent whose draw rows are back-filled on join, so the opener's seat is fixed up front and the existing open-game pre-move is preserved (no reseating, no play-gating). Admin `/_gm/games/:id` gains the recorded draw list + a simple **step-by-step board replay** (`ReplayTimeline`). | owner ad-hoc | **done** |
+| SB | Single Telegram bot + per-user variant preferences: the two per-language bots collapse into **one** (drop `accounts.service_language`, `supported_languages`, the `*_EN`/`*_RU` env vars and game-language push routing — the single bot renders in the recipient's `preferred_language`); New Game variant gating moves to a profile **`variant_preferences`** set (default Erudit only, Erudit-first, server-enforced on the caller's auto-match/vs-AI/invitation-create paths, an invited friend may accept any variant); env vars collapse to unsuffixed `TELEGRAM_BOT_TOKEN`/`TELEGRAM_GAME_CHANNEL_ID`/`VITE_TELEGRAM_LINK`/`VITE_TELEGRAM_GAME_CHANNEL_NAME` and `GATEWAY_DEFAULT_SUPPORTED_LANGUAGES` is removed; wire drops `service_language`/`supported_languages` (Session, ValidateInitDataResponse) + the push `language` routing field and adds `variant_preferences` to Profile/UpdateProfile. | owner ad-hoc | **done** |
 | → | Stage 18 — prod contour deploy | — | see [`PLAN.md`](PLAN.md) |
 
 ## Key findings (these reshaped the raw list — read before starting a phase)