Fix subscription cleanup to prevent zombie subscriptions

[ratkokostov7]

When WebSocket connections closed under high traffic, subscriptions weren't cleaned up immediately after closing the session. This created "zombie subscriptions" that accumulated in Redis.

• Immediate cleanup: Remove subscriptions instantly when connections close using batched Redis operations
• Configurable safety net: Keep periodic cleanup (now every 10 minutes) to catch any missed orphans
• Performance improvement: Eliminates need to scan/filter large subscription sets