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.

PRERELEASE.md

@@ -26,6 +26,7 @@ the edge before prod. Each phase maps back to the owner's raw pre-release TODO l
 | R7 | Final stress run + tuning | 9b | **done** |
 | UI | Tab-bar navigation redesign (drop the hamburger) | owner ad-hoc | **done** |
 | MW | "Multiple words per turn" rule for Russian games (engine v1.1.0) | owner ad-hoc | **done** |
+| OW | Open auto-match: enter the game at once and wait inside it (robot after 90–180 s) | 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)
@@ -69,6 +70,14 @@ the edge before prod. Each phase maps back to the owner's raw pre-release TODO l
 - **Rate-abuse (TODO 8):** metric + Grafana + admin view **plus a conservative auto-flag** —
   a *soft, reversible* "suspected high-rate" marker for operator review, tunable threshold,
   **no auto-ban**.
+- **Open auto-match (owner ad-hoc):** a quick game **enters a real game at once and waits inside
+  it** (status `open`, the opponent seat empty); a second human searching the same variant+rule
+  joins it, or a robot fills it after a **90 s + random 0–90 s** wait, pushing the in-app
+  **opponent_joined** event. While open, the starter may move on their turn but resign, chat and
+  nudge are disabled, and the lobby + opponent card read "searching for opponent". Matchmaking is
+  now **DB-backed open games** — the in-memory pool, `lobby.poll` and `lobby.cancel` are gone. The
+  schema is edited in the baseline (no prod data); `game_players.account_id` is nullable for the
+  empty seat.
 
 ## Phases