feat: Pub/Sub (SUBSCRIBE, PUBLISH, UNSUBSCRIBE, PSUBSCRIBE)

[zulerne]

Pub/Sub is needed for the messenger project — message delivery between service instances via Redis.

Commands

• `SUBSCRIBE [channel ...]` — subscribe to channels
• `PUBLISH ` — publish message, returns subscriber count
• `UNSUBSCRIBE [channel ...]` — unsubscribe from channels
• `PSUBSCRIBE ` — pattern-based subscription

Implementation

• Connection enters "subscriber mode" after SUBSCRIBE — can only receive messages and run (P)SUBSCRIBE/(P)UNSUBSCRIBE
• Hub pattern: central registry of channels → subscriber connections
• Pattern matching for PSUBSCRIBE (glob-style)
• Requires changes to server connection loop to support push messages