Fix monitoring test panic and goroutine leaks

Two critical fixes to prevent test timeouts:

1. Nil map panic in TestPollPVEInstanceUsesRRDMemUsedFallback:
   - Test monitor was missing nodeLastOnline map initialization
   - Panic occurred when pollPVEInstance tried to update nodeLastOnline[nodeID]
   - Caused deadlock when panic recovery tried to acquire already-held mutex
   - Added nodeLastOnline: make(map[string]time.Time) to test monitor

2. Alert manager goroutine leak in Docker tests:
   - newTestMonitor() created alert manager but never stopped it
   - Background goroutines (escalationChecker, periodicSaveAlerts) kept running
   - Added t.Cleanup(func() { m.alertManager.Stop() }) to test helper

These fixes resolve the 10+ minute test timeouts in CI workflows.

Related to workflow run 19281508603.

internal/monitoring/monitor_docker_test.go

@@ -13,7 +13,7 @@ import (
 func newTestMonitor(t *testing.T) *Monitor {
 	t.Helper()
 
-	return &Monitor{
+	m := &Monitor{
 		state:                  models.NewState(),
 		alertManager:           alerts.NewManager(),
 		removedDockerHosts:     make(map[string]time.Time),
@@ -21,6 +21,8 @@ func newTestMonitor(t *testing.T) *Monitor {
 		dockerTokenBindings:    make(map[string]string),
 		dockerMetadataStore:    config.NewDockerMetadataStore(t.TempDir()),
 	}
+	t.Cleanup(func() { m.alertManager.Stop() })
+	return m
 }
 
 func TestApplyDockerReportGeneratesUniqueIDsForCollidingHosts(t *testing.T) {

internal/monitoring/monitor_memory_test.go

@@ -191,6 +191,7 @@ func TestPollPVEInstanceUsesRRDMemUsedFallback(t *testing.T) {
 		dlqInsightMap:        make(map[string]*dlqInsight),
 		authFailures:         make(map[string]int),
 		lastAuthAttempt:      make(map[string]time.Time),
+		nodeLastOnline:       make(map[string]time.Time),
 	}
 	defer mon.alertManager.Stop()
 	defer mon.notificationMgr.Stop()