feat(lobby): enter the game immediately and wait for the opponent inside it

Quick auto-match no longer waits on a separate screen: Enqueue opens a real game seating the caller with an empty opponent seat (new game status 'open') and the player enters it at once. A second human searching the same variant+rule joins that open game; otherwise a background reaper seats a robot after a 90s + random 0-90s wait, pushing a new in-app opponent_joined event that fills the opponent card and re-enables resign and chat in place.

Matchmaking state is now the open games in the database (the in-memory pool, lobby.poll and lobby.cancel are gone), serialised by a per-bucket advisory lock. While a game is open the starter may move on their turn, but resign, chat and nudge are refused; the lobby and opponent card show "searching for opponent".

Schema edited in the baseline (no prod data): 'open' status, nullable game_players.account_id for the empty seat, and a games.open_deadline_at stamp; jet code regenerated.

backend/internal/server/handlers_user.go

@@ -133,13 +133,15 @@ func (s *Server) handleGameState(c *gin.Context) {
 	c.JSON(http.StatusOK, dto)
 }
 
-// enqueueRequest joins the per-variant auto-match pool under a per-turn word rule.
+// enqueueRequest enters per-variant auto-match under a per-turn word rule.
 type enqueueRequest struct {
 	Variant              string `json:"variant"`
 	MultipleWordsPerTurn bool   `json:"multiple_words_per_turn"`
 }
 
-// handleEnqueue joins the auto-match pool for a variant.
+// handleEnqueue enters the caller into auto-match for a variant and returns the game
+// they land in immediately: a freshly opened game awaiting an opponent, or another
+// player's open game they just joined. The client navigates straight into the game.
 func (s *Server) handleEnqueue(c *gin.Context) {
 	uid, ok := userID(c)
 	if !ok {
@@ -168,39 +170,6 @@ func (s *Server) handleEnqueue(c *gin.Context) {
 	c.JSON(http.StatusOK, dto)
 }
 
-// handleCancel removes the caller from the auto-match pool (and drops any pending
-// matched result), so a cancelled quick-match neither blocks a re-queue nor later
-// surfaces a robot-substituted game the player abandoned. It is idempotent: cancelling
-// when not queued is a no-op success.
-func (s *Server) handleCancel(c *gin.Context) {
-	uid, ok := userID(c)
-	if !ok {
-		abortBadRequest(c, "missing identity")
-		return
-	}
-	s.matchmaker.Cancel(c.Request.Context(), uid)
-	c.Status(http.StatusNoContent)
-}
-
-// handlePoll reports whether the caller has been paired since queueing.
-func (s *Server) handlePoll(c *gin.Context) {
-	uid, ok := userID(c)
-	if !ok {
-		abortBadRequest(c, "missing identity")
-		return
-	}
-	res, err := s.matchmaker.Poll(c.Request.Context(), uid)
-	if err != nil {
-		s.abortErr(c, err)
-		return
-	}
-	dto := matchDTOFrom(res)
-	if dto.Game != nil {
-		s.fillSeatNames(c.Request.Context(), dto.Game, map[string]string{})
-	}
-	c.JSON(http.StatusOK, dto)
-}
-
 // chatPostRequest posts a per-game chat message.
 type chatPostRequest struct {
 	Body string `json:"body"`