fix(store): replace panic with error return, handle unmarshal errors

generateToken() now returns (string, error) instead of panicking on
crypto/rand failure. All json.Unmarshal calls for alert settings now
check and propagate errors instead of silently ignoring them.

Adds Close() to Store interface for graceful shutdown support.
Skips malformed notification entries during Kuma import.

internal/metrics/prometheus_test.go

@@ -64,6 +64,7 @@ func (m *mockStore) DeleteMaintenanceWindow(int) error                   { retur
 func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)            { return false, nil }
 func (m *mockStore) GetPreference(string) (string, error)                { return "", nil }
 func (m *mockStore) SetPreference(string, string) error                  { return nil }
+func (m *mockStore) Close() error                                        { return nil }
 
 func TestMetricsHandler(t *testing.T) {
 	ms := &mockStore{