feat: expose match date on volunteer list and detail endpoints

[nadavosa]

Summary

Volunteer cards in the dashboard need to show when a volunteer was matched. The volunteer list and detail endpoints currently do not return a match date. Add the match date to the response so the FE can display it.

Relates to

need4deed-org/fe#423

Acceptance criteria

• GET /volunteer and GET /volunteer/:id include a matchDate (or equivalent) field in the response when a match exists

[arturasmckwcz]

@nadavosa
ATM volunteer entity would need additional property storing timestamp for persisting moment of matching, but it would be overwritten by the next matched opportunity if any. is that okay?
also we've got intermediate many-to-many entity opportunity_volunteer which has got create and update timestamps. so it's property updated_at would indicate matching moment before the next state: active