feat(lobby): cap simultaneous quick games at 10

Limit a player to 10 active quick games (auto-match + AI); friend games created
by invitation are not counted. At the cap the backend refuses both new-game
entry points — quick enqueue and invitation creation — with 409
game_limit_reached, while accepting an incoming invitation stays allowed, so
friend games are capped from the other end. The lobby disables "New Game" and
shows a low-emphasis notice, driven by a new at_game_limit flag on games.list
(no per-event payload: a turn change does not move the count, and the lobby
already re-fetches games.list on entry and every game event).

- game.MaxActiveQuickGames + Store/Service.CountActiveQuickGames (active/open
  seats, no game_invitations row; hidden games still count -> dedicated count)
- Server.ensureUnderGameLimit gating handleEnqueue + handleCreateInvitation;
  game.ErrGameLimitReached -> 409 game_limit_reached
- FB GameList.at_game_limit (regenerated Go + TS) through the gateway transcode
  and UI codec; gameListDTO + lobbycache snapshot + Lobby.svelte + i18n
- tests: integration count rule + HTTP gate + accept bypass; server error map;
  gateway transcode round-trip; UI codec + lobbycache unit; e2e gamelimit
- docs: PRERELEASE (GL), FUNCTIONAL(+ru), ARCHITECTURE 8, UI_DESIGN, backend README

backend/README.md

@@ -33,7 +33,12 @@ real game seating the caller with an **empty opponent seat** (status `open`) or,
 another player already waits for the same variant and per-turn word rule, seats the
 caller into that open game and starts it — and
 friend-game invitations (invite → accept, starting a 2–4 player game once every
-invitee accepts). `internal/social` owns the friend graph (request/accept),
+invitee accepts). A **simultaneous-game cap** (`game.MaxActiveQuickGames` = 10) limits a
+player's active quick games — status `active`/`open`, excluding invitation-linked friend
+games (`game.Service.CountActiveQuickGames`); the server refuses `lobby/enqueue` and
+`invitations` creation with **409 `game_limit_reached`** at the cap (accepting an invitation
+is exempt), and the `games.list` response carries an `at_game_limit` flag for the lobby.
+`internal/social` owns the friend graph (request/accept),
 per-user blocks, and per-game chat with nudges folded in as a message kind; chat
 messages are length-capped, content-filtered (no links/emails/phone numbers,
 including obfuscated forms) and stored with the sender's IP. `internal/account`