fix(population): exclude nested circles from population counts

[cristianscheid]

• Related to: Wrong user count when using teams as team owners #2126

Summary

Fixes team population calculation to exclude nested teams from member counts.

Changes

• When a team is added as member of another team, only users within the nested team are counted (team itself and duplicates are not counted)
• When team members are added/removed, all parent circles (circles containing the updated circle) are also updated
• When a team is deleted, all teams that contained that team are updated to reflect the removal
• Added populationInherited field to Circle::jsonSerialize() alongside existing population for frontend usage in follow-up PR without breaking current frontend

Frontend follow-up needed

This PR addresses only the backend part of the issue. A follow-up PR in nextcloud/contacts is required to fix inconsistent count display:

• Currently: uses population for member count on page load, but switches to member list length when clicking a team
• Expected: consistently use the same count throughout the interface (populationInherited, added in this PR)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[cristianscheid]

/backport to stable31

[cristianscheid]

/backport to stable32