feat(mgmt): add SSE endpoint for live cluster topology events

[hyp3rd]

Introduce GET /cluster/events on the management HTTP port, streaming members and heartbeat Server-Sent Events to subscribers in real time, replacing the monitor's 2-second poll cadence with a push-based model.

Key changes:

• New internal/eventbus package: in-process fan-out broadcaster with drop-on-full semantics (slow consumers never backpressure the SWIM heartbeat loop) and context-driven subscription reaping.
• Membership.OnStateChange observer hook fires after every mutation (Upsert/Mark/Remove) with the lock already released, publishing a full membership snapshot onto the event bus.
• 1 Hz heartbeat event publisher aligned with the SWIM interval.
• defaultWriteTimeout changed from 5s to 0 on the mgmt server to prevent fasthttp from force-closing the long-lived SSE stream.
• HyperCache.EventBus() accessor exposes the bus to the SSE handler; non-distributed backends return nil (503 fallback).
• Connect-time frames deliver current snapshots so fresh subscribers see topology immediately without waiting for the next mutation.

Includes unit tests for eventbus and membership observers, plus an integration test (TestManagementHTTP_ClusterEvents) that reads frames for 6s past the historic 5s WriteTimeout deadline to pin the regression.