feat: add support for setting preview avatar for circle

[cristianscheid]

• Resolves: Support associating a "preview" picture to teams #2433

Summary

This PR so far focuses on the backend part of implementation.

• Adds new property avatar for circle model.
• Adds route to set avatar: POST ocs/v2.php/apps/circles/circles/<circle_id>/avatar
• Adds route to fetch avatar: GET ocs/v2.php/apps/circles/circles/<circle_id>/avatar
• Adds route to remove avatar: DELETE ocs/v2.php/apps/circles/circles/<circle_id>/avatar

TODO

Implement frontend.

Test plan

Run migration to create avatar column on circles_circle table.

occ migrations:execute circles 34000Date20260407183611

Set avatar (from terminal):

curl -X POST \
  -u "admin:admin" \
  -H "OCS-APIRequest: true" \
  -F "file=@/home/<username>/<folder>/mock_image.jpg" \
  "http://nextcloud.local/ocs/v2.php/apps/circles/circles/<circle_id>/avatar"

Get avatar (from browser):

http://nextcloud.local/ocs/v2.php/apps/circles/circles/<circle_id>/avatar

Remove avatar (from terminal):

curl -X DELETE \
  -u "admin:admin" \
  -H "OCS-APIRequest: true" \
  "http://nextcloud.local/ocs/v2.php/apps/circles/circles/<circle_id>/avatar"

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)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[cristianscheid]

@nextcloud/designers would appreciate some input on the UI for uploading and updating avatar images for circles. One idea that came up was to make it similar to the user personal settings. Does that sound like a good idea?

Contacts app	User's personal settings

[kra-mo]

One idea that came up was to make it similar to the user personal settings. Does that sound like a good idea?

I actually thought that the difference between these two buttons was quite unclear so I wanted to give them more explanation text, see nextcloud/server#57008.

With that in mind, I think we could just add these 2/3 (depending on whether a picture is uploaded) buttons beneath the name and description in Edit mode.

Also, now that I'm looking at it, to make it less busy and more focused especially with these buttons, would it also be possible for the rest of the content to disappear while in edit mode? Including the Leave Team button, creating new items, etc.