fix(games): skip upfront bet deduction for per-round betting games

Games with getActionCost (like blackjack) handle bet deductions per-round
via place_bet actions. The old deductBetsAndStart was also charging at game
start, causing double-deduction: wins netted zero and losses doubled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/src/games/GameServer.ts

@@ -395,6 +395,18 @@ export class GameServer {
     ): Promise<void> {
         const room = this.roomManager.getRoom(roomId);
         if (!room || room.betsPending) return;
+
+        // Games with getActionCost handle per-round betting themselves (e.g., blackjack).
+        // Skip the upfront deduction — just start the game.
+        const plugin = gameRegistry.get(room.gameSlug);
+        if (plugin?.getActionCost) {
+            const startResult = this.roomManager.startGame(roomId);
+            if (!startResult.ok) {
+                triggeringWs.send(JSON.stringify({ type: "ERROR", message: startResult.error }));
+            }
+            return;
+        }
+
         room.betsPending = true;
 
         const uniquePlayers = [...new Set(playerIds)];