Feature(messenger): messagePending wakeup + socket cleanup

[tada5hi]

📍 Part of roadmap #1710

Part of the Message Broker Rewrite (roadmap #1710). Reduce the socket layer to a single job and delete the generic relay/presence (scope decision: single-purpose broker).

Scope

• Delete the generic send relay, presence (*Connected/*Disconnected), and connection-subscription controllers + their messenger-kit types.
• MessageSubscriber.afterInsert publishes a payload-free messagePending domain event (per repo convention: subscribers publish, controllers stay thin).
• One publish drives both the socket wakeup (recipient room, cross-instance via Redis adapter) and resolution of any parked long-poll GET.
• Wakeup fires on every write path (single send + broadcast fan-out). Tests.

Blocked by: Phase 1. Roadmap & design: #1710.