feat: add updated_at timestamp in *MembershipData

[amandazhuyilan]

Description

AAI-552: add updated_at timestamp in *MembershipData

Changes

• Added updated_at timestamp to PlatformMembershipData and GroupMembershipData and populate it from the models’ get_data() methods. This surfaces action timestamps directly in API responses.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.