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/eventbus.go

@@ -10,6 +10,12 @@ type EventBus struct {
 	mu          sync.RWMutex
 	global      chan Event
 	bufferSize  int
+	globalSubs  []globalSub
+}
+
+type globalSub struct {
+	ch   chan Event
+	done chan struct{}
 }
 
 func NewEventBus(bufferSize int) *EventBus {
@@ -17,6 +23,7 @@ func NewEventBus(bufferSize int) *EventBus {
 		subscribers: make(map[int64]map[chan Event]struct{}),
 		global:      make(chan Event, bufferSize),
 		bufferSize:  bufferSize,
+		globalSubs:  nil,
 	}
 }
 
@@ -43,20 +50,42 @@ func (eb *EventBus) Subscribe(jobID int64) (chan Event, func()) {
 }
 
 func (eb *EventBus) SubscribeGlobal() (chan Event, func()) {
-	eb.mu.RLock()
 	ch := make(chan Event, eb.bufferSize)
-	eb.mu.RUnlock()
+	done := make(chan struct{})
+	eb.mu.Lock()
+	eb.globalSubs = append(eb.globalSubs, globalSub{ch: ch, done: done})
+	eb.mu.Unlock()
 	go func() {
-		for evt := range eb.global {
+		defer func() {
+			if r := recover(); r != nil {
+				slog.Error("SubscribeGlobal goroutine panicked", "reason", r)
+			}
+			close(ch)
+		}()
+		for {
 			select {
-			case ch <- evt:
-			default:
-				slog.Warn("global event subscriber buffer full, dropping event", "type", evt.Type)
+			case evt := <-eb.global:
+				select {
+				case ch <- evt:
+				default:
+					slog.Warn("global event subscriber buffer full, dropping event", "type", evt.Type)
+				}
+			case <-done:
+				return
 			}
 		}
-		close(ch)
 	}()
-	return ch, func() { close(ch) }
+	return ch, func() {
+		close(done)
+		eb.mu.Lock()
+		for i, s := range eb.globalSubs {
+			if s.ch == ch {
+				eb.globalSubs = append(eb.globalSubs[:i], eb.globalSubs[i+1:]...)
+				break
+			}
+		}
+		eb.mu.Unlock()
+	}
 }
 
 func (eb *EventBus) Publish(evt Event) {