bug: volunteers not shown under their NGO despite being connected via opportunities

[nadavosa]

Problem

On the NGO/agent profile page, the "Ehrenamtliche" (Volunteers) section shows 0 in every tab (Ausstehend/Zugeordnet/Aktiv/Vergangen) and "Noch keine Einträge in dieser Kategorie" — even when the NGO has an opportunity that has a volunteer assigned to it.

Volunteers are linked to opportunities, and opportunities are linked to NGOs, but that chain isn't being used to populate the NGO's volunteer list — so from the NGO profile it looks like there are no volunteers at all, when there actually is at least one assigned via an opportunity.

Steps to reproduce

1. Open an NGO/agent profile that has an opportunity with an assigned/matched volunteer (e.g. "Begleitung zum Termin beim Facharzt Onkologie")
2. Check the "Ehrenamtliche" section tabs (Ausstehend/Zugeordnet/Aktiv/Vergangen)
3. All tabs show 0 and no entries, despite the volunteer being connected via the opportunity

Screenshot

See attached screenshot — "Ehrenamtliche" section with all counts at 0, and the "Ehrenamtliche Tätigkeiten" section right below showing an opportunity that does have a volunteer relationship.

Expected

Volunteers connected to the NGO via one of its opportunities should show up under the appropriate tab in the NGO's "Ehrenamtliche" section.