fix: reset SSE inactivity timeout on each broadcast

[jakebromberg]

Summary

• Bug: The 5-minute SSE inactivity timeout in ServerEventsManager was set once at client registration and never reset, causing active clients receiving broadcasts/dispatches to be disconnected after exactly 5 minutes regardless of activity.
• Fix: Store timeout handles per client in a clientTimeouts map. Added a resetTimeout() method that clears and restarts the timer, called on every successful broadcast and dispatch write. Timeouts are also cleaned up in unsubAll and the close handler.
• Tests: Added 4 unit tests covering idle disconnect, timeout reset on broadcast, timeout reset on dispatch, and eventual disconnect after a reset window elapses.

Fixes #33

Test plan

• npm run test:unit -- tests/unit/utils/serverEvents.test.ts — all 4 tests pass
• Manual: connect an SSE client, leave idle > 5 min → verify disconnect
• Manual: connect an SSE client, send broadcasts within 5-min windows → verify it stays connected

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