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.

ui/src/game/Chat.svelte

@@ -8,6 +8,7 @@
     myId,
     busy,
     myTurn = false,
+    waiting = false,
     nudgeOnCooldown = false,
     onsend,
     onnudge,
@@ -20,6 +21,9 @@
     // hurry); on the opponent's turn only the nudge button shows. While the hourly nudge
     // cooldown is active the nudge is disabled with an "awaiting reply" caption.
     myTurn?: boolean;
+    // waiting is true while an auto-match game still has no opponent: both send and nudge
+    // are disabled (there is no one to message or hurry yet).
+    waiting?: boolean;
     nudgeOnCooldown?: boolean;
     onsend: (text: string) => void;
     onnudge: () => void;
@@ -56,11 +60,11 @@
         bind:value={text}
         onkeydown={(e) => e.key === 'Enter' && send()}
       />
-      <button class="iconbtn" onclick={send} disabled={busy || !connection.online} aria-label={t('chat.send')}>⬆️</button>
+      <button class="iconbtn" onclick={send} disabled={busy || waiting || !connection.online} aria-label={t('chat.send')}>⬆️</button>
     {:else}
       <!-- A flex:1 caption keeps the nudge pinned right whether or not the cooldown text shows. -->
       <span class="cooldown">{nudgeOnCooldown ? t('chat.awaitingReply') : ''}</span>
-      <button class="iconbtn" onclick={onnudge} disabled={busy || nudgeOnCooldown || !connection.online} aria-label={t('chat.nudgeAction')}>🛎️</button>
+      <button class="iconbtn" onclick={onnudge} disabled={busy || waiting || nudgeOnCooldown || !connection.online} aria-label={t('chat.nudgeAction')}>🛎️</button>
     {/if}
   </div>
 </div>