Is your feature request related to a problem? Please describe.

In Icinga DB Web, SLA history (sla_history_state / sla_history_downtime) is filterable by hostgroup via the Host model. However, the same relations are missing from the Service model, making it impossible to filter SLA history by servicegroup when working with services.

Describe the solution you'd like

Add sla_history_state and sla_history_downtime relations to the Service model, mirroring what was done for the Host model.

Additional context

Refs #1323 — Add support for filtering SLA history by hostgroup
Refs #1324 — Add hostgroup filtering support for SLA history

This extends the same approach to the Service model, so that SLA history can be filtered by servicegroup as well.