fix(cors): allow the Idempotency-Key header

The EnakGame client runs at its game_url, another origin than the API, so the
browser asks before POST /customer/enakgame/sessions. Idempotency-Key was not in
Access-Control-Allow-Headers, so the preflight refused it and a play could not
start. X-Idempotency-Key, the older name the backend also reads, is allowed too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/middleware/cors.go

@@ -12,7 +12,7 @@ func CORS() gin.HandlerFunc {
 		}
 		c.Header("Access-Control-Allow-Origin", origin)
 		c.Header("Access-Control-Allow-Credentials", "true")
-		c.Header("Access-Control-Allow-Headers", "Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization, accept, origin, Cache-Control, X-Requested-With")
+		c.Header("Access-Control-Allow-Headers", "Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization, accept, origin, Cache-Control, X-Requested-With, Idempotency-Key, X-Idempotency-Key")
 		c.Header("Access-Control-Allow-Methods", "POST, OPTIONS, GET, PUT, DELETE")
 
 		if c.Request.Method == "OPTIONS" {