Add LastSeenAt tracking for user activity metrics

[tjementum]

Summary & Motivation

Add a dedicated LastSeenAt property to the User entity to accurately track when users were last active, replacing the unreliable ModifiedAt field which changes on any user update (role changes, profile edits, etc.). This provides meaningful activity data for the "Active users" dashboard metric, which now correctly counts users with activity within the last 30 days.

• Add LastSeenAt nullable property to User entity, updated during login, token refresh, tenant switch, and signup completion
• Create database schema migration to add the LastSeenAt column
• Introduce the first data migration using IDataMigration to populate existing confirmed users with their ModifiedAt or CreatedAt values
• Update user management UI to display "Last seen" instead of "Modified" in both the user table and profile side pane
• Change the GetUserSummary query to use LastSeenAt for calculating active users instead of ModifiedAt
• Add backend test for user summary to verify active user counting based on LastSeenAt
• Add E2E test coverage to verify LastSeenAt is preserved during role changes and user restore operations

This is the first usage of IDataMigration in PlatformPlatform. Unlike schema migrations which run during CI/CD before code deployment (requiring old code to work with the new schema), data migrations run when the Worker starts after deployment. Since API and Worker deploy in parallel with independent rolling updates, old API instances may still write data after the data migration completes, so data migrations should be designed to handle this timing window.

Checklist

• I have added tests, or done manual regression tests
• I have updated the documentation, if necessary

[linear]

PP-751 Tast last seen on user and display in user overview

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
58.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Approve Database Migration account-management database on stage

The following pending migration(s) will be applied to the database when approved:

• AddUserLastSeenAt (20260106160000_AddUserLastSeenAt)

Migration Script

BEGIN TRANSACTION;
IF NOT EXISTS (
    SELECT * FROM [__EFMigrationsHistory]
    WHERE [MigrationId] = N'20260106160000_AddUserLastSeenAt'
)
BEGIN
    ALTER TABLE [Users] ADD [LastSeenAt] datetimeoffset NULL;
END;

IF NOT EXISTS (
    SELECT * FROM [__EFMigrationsHistory]
    WHERE [MigrationId] = N'20260106160000_AddUserLastSeenAt'
)
BEGIN
    INSERT INTO [__EFMigrationsHistory] ([MigrationId], [ProductVersion])
    VALUES (N'20260106160000_AddUserLastSeenAt', N'10.0.1');
END;

COMMIT;
GO