fix: clean up activeShards on reconnection failure

Remove stale entries from the activeShards map when
reconnectWithBackoff exhausts all attempts or detects room closure.

Made-with: Cursor

backend/utils/ecast-shard-client.js

@@ -378,6 +378,7 @@ class EcastShardClient {
             reason: 'room_closed',
             finalPlayerCount: this.playerCount,
           });
+          activeShards.delete(`${this.sessionId}-${this.gameId}`);
           return false;
         }
 
@@ -397,6 +398,7 @@ class EcastShardClient {
         reason: 'connection_failed',
         finalPlayerCount: this.playerCount,
       });
+      activeShards.delete(`${this.sessionId}-${this.gameId}`);
       return false;
     } finally {
       this.reconnecting = false;