Get resource-specific last seen timestamps for multiple users in a project

Returns last seen data for specified users within a project. Requires `canGetProjectMembership`. Unknown user ids are silently omitted from `results`; known users with no record get a null `lastSeen`.

curl -X POST "https://example.com/memberships/project/497f6eca-6276-4993-bfeb-53cbbbba6f08/last-seen" \  -H "Content-Type: application/json" \  -d '{    "userIds": [      "507f1f77bcf86cd799439011"    ]  }'

{  "results": [    {      "userId": "507f1f77bcf86cd799439011",      "resourceId": "507f1f77bcf86cd799439012",      "resourceType": "project",      "lastSeen": "2025-10-24T10:30:00.000Z"    }  ]}