SSE polls at 5 Hz per connection #289

Location: events.py:60
Severity: MED

Every open stream serializes the full job state to JSON 5×/second and compares strings. At the 200-connection cap that is ~1,000 json.dumps/s of pure overhead.
Bump a per-job version counter in _set and only serialize when it changes; or drop the poll to 3–4 Hz (imperceptible in the UI).

From the health report. Part of #274 (Phase 3 — Performance).