Fix backup indicator being reset when VMs/Containers are re-polled

UpdateVMsForInstance and UpdateContainersForInstance were replacing
guest data without preserving the LastBackup field that was populated
by SyncGuestBackupTimes. This caused backup indicators to always show
"no backup found" since the LastBackup would be wiped every time
guests were polled (which happens more frequently than backup polling).

Now both functions preserve LastBackup from existing data when the
incoming guest data has a zero value.

Related to #762

internal/api/security_test.go

@@ -616,6 +616,31 @@ func TestGetLockoutInfo(t *testing.T) {
 			t.Error("expected lockedUntil to be set")
 		}
 	})
+
+	t.Run("expired lockout returns zeros", func(t *testing.T) {
+		resetFailedLogins()
+		identifier := "expired-lockout-user"
+
+		// Directly set an expired lockout entry
+		failedMu.Lock()
+		failedLogins[identifier] = &FailedLogin{
+			Count:       maxFailedAttempts,
+			LastAttempt: time.Now().Add(-time.Hour),
+			LockedUntil: time.Now().Add(-time.Minute), // Expired
+		}
+		failedMu.Unlock()
+
+		attempts, lockedUntil, isLocked := GetLockoutInfo(identifier)
+		if attempts != 0 {
+			t.Errorf("attempts = %d, want 0 for expired lockout", attempts)
+		}
+		if !lockedUntil.IsZero() {
+			t.Errorf("lockedUntil = %v, want zero time for expired lockout", lockedUntil)
+		}
+		if isLocked {
+			t.Error("expected isLocked = false for expired lockout")
+		}
+	})
 }
 
 func TestResetLockout(t *testing.T) {

internal/models/models.go

@@ -1083,6 +1083,14 @@ func (s *State) UpdateVMsForInstance(instanceName string, vms []VM) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
+	// Build a lookup of existing VMs for this instance to preserve LastBackup
+	existingByVMID := make(map[int]VM)
+	for _, vm := range s.VMs {
+		if vm.Instance == instanceName {
+			existingByVMID[vm.VMID] = vm
+		}
+	}
+
 	// Create a map of existing VMs, excluding those from this instance
 	vmMap := make(map[string]VM)
 	for _, vm := range s.VMs {
@@ -1091,8 +1099,11 @@ func (s *State) UpdateVMsForInstance(instanceName string, vms []VM) {
 		}
 	}
 
-	// Add or update VMs from this instance
+	// Add or update VMs from this instance, preserving LastBackup from existing data
 	for _, vm := range vms {
+		if existing, ok := existingByVMID[vm.VMID]; ok && vm.LastBackup.IsZero() {
+			vm.LastBackup = existing.LastBackup
+		}
 		vmMap[vm.ID] = vm
 	}
 
@@ -1169,6 +1180,14 @@ func (s *State) UpdateContainersForInstance(instanceName string, containers []Co
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
+	// Build a lookup of existing containers for this instance to preserve LastBackup
+	existingByVMID := make(map[int]Container)
+	for _, ct := range s.Containers {
+		if ct.Instance == instanceName {
+			existingByVMID[ct.VMID] = ct
+		}
+	}
+
 	// Create a map of existing containers, excluding those from this instance
 	containerMap := make(map[string]Container)
 	for _, container := range s.Containers {
@@ -1177,8 +1196,11 @@ func (s *State) UpdateContainersForInstance(instanceName string, containers []Co
 		}
 	}
 
-	// Add or update containers from this instance
+	// Add or update containers from this instance, preserving LastBackup from existing data
 	for _, container := range containers {
+		if existing, ok := existingByVMID[container.VMID]; ok && container.LastBackup.IsZero() {
+			container.LastBackup = existing.LastBackup
+		}
 		containerMap[container.ID] = container
 	}