Show which Teams a user belongs to #2955

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
V34

Is your feature request related to a problem? Please describe.
There's no way, in the UI, to see which Teams a given user belongs to.
Checking means opening every Team one by one and searching its member
list — doesn't scale, and misses private Teams the checker isn't in.

This also blocks delegated admins: someone holding the "Users" privilege
on /settings/admin/admindelegation can manage a user's account but has
no way to see or explain that user's Team memberships.

Describe the solution you'd like
A simple view listing a user's Teams (name + role: member/moderator/
admin/owner), e.g. as a section on the user's row in Settings > Users, or
a lookup accessible from elsewhere.

No new delegation privilege needed — gate it behind the existing "Users"
privilege or "Team folders". Whoever already holds one of those gets
the view; full admins keep it by default.

Describe alternatives you've considered

• Browsing every Team manually — impractical, misses private Teams.
• Querying oc_circles_member directly / OCS API — works for full admins
  with DB/CLI access, not for delegated admins.
• A third-party app (but Beta) fills part of this gap for full admins (see
  https://help.nextcloud.com/t/overview-of-all-user-created-teams-circles/241465)

Additional context
Related to #13478 (unifying Users/Groups/Teams) ?
and #2936 (filtering which Circles a user sees on their own Teams home) — this is the admin-facing, cross-user equivalent.