fix: EventBus panic on SSE disconnect + JWT secret persistence + recover() guards

- eventbus.go: Fix send-on-closed-channel panic in SubscribeGlobal by
  using a done channel; add recover() in fan-out goroutine; track active
  global subs for proper cleanup on unsubscribe
- config.go: Persist JWT secret to $DATA_DIR/.jwt_secret instead of
  regenerating a random one on every restart (which invalidated all sessions)
- handlers_ws.go: Replace time.After with time.Ticker to fix timer leak in
  SSE keepalive loop
- handlers_jobs.go: Add recover() in fire-and-forget job goroutine; fix nil
  pointer deref when GetByID fails after job creation
- handlers_machines.go: Add recover() in ProbeAllMachines goroutine
- scheduler.go: Add recover() in scheduled job run goroutine
- engine.go: Add recover() in per-machine probe goroutines

internal/syncengine/engine.go

@@ -368,7 +368,12 @@ func (e *Engine) ProbeAllMachines() {
 		sem <- struct{}{}
 		go func(m *models.Machine) {
 			defer wg.Done()
-			defer func() { <-sem }()
+			defer func() {
+				if r := recover(); r != nil {
+					slog.Error("probe goroutine panicked", "machine_id", m.ID, "panic", r)
+				}
+				<-sem
+			}()
 
 			ctx, cancel := context.WithTimeout(context.Background(), probeTimeout)
 			defer cancel()