feat: send periodic SSE heartbeats to reap half-open connections

[LounisBou]

Idle SSE subscribers whose client dies silently (mobile drop, laptop sleep, NAT/proxy timeout) never emit a stream close, so they leak in the transport's topic index (~kernel keepalive, ~2h) and keep getting re-matched on every publish, wasting CPU.

This adds a periodic SSE heartbeat (:\n SSE comment) per subscription. It (a) keeps live connections open through reverse-proxy idle timeouts and (b) forces a TCP write on half-open peers so the kernel detects the dead connection and emits close, triggering the existing unsubscribe path.

• New heartbeat_interval hub option (default 40s), configurable via HEARTBEAT_INTERVAL env; 0 disables.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (babfcd2) to head (33613c5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##                main       #50   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       165       165           
===========================================
  Files             27        27           
  Lines            437       443    +6     
===========================================
+ Hits             437       443    +6     

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[bpolaszek]

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