Set Team Photo From Result

Promote one of a deferred-judging result's submitted photos to the team's official photo.

Gated by rally_settings.allow_photo_as_team_photo so an admin can turn this capability off event-wide. The chosen URL must already be one of the result's own media_urls (already stored in R2) to prevent staff from pointing a team's photo at an arbitrary URL, and — like the capture route — scoped to the caller's own post: proving some assignment was never the same as proving this one.