## Fix last seen timestamp in `/_synapse/admin/v2/users` response
Fixes#18955
The last seen timestamps contained in `/_synapse/admin/v2/users`
responses were computed as follows:
```sql
[...]
LEFT JOIN (
SELECT user_id, MAX(last_seen) AS last_seen_ts
FROM user_ips GROUP BY user_id
) ls ON u.name = ls.user_id
[...]
```
4367fb2d07/synapse/storage/databases/main/__init__.py (L302C1-L305C44)
This leads to empty timestamps (as in: user was never seen) if users are
inactive for longer than
[`user_ips_max_age`](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#user_ips_max_age).
The fix is quite trivial: Use the `devices` table, as this one also
contains last seen timestamps but is *not* periodically purged.
We are using this for automatic user account deletion (via
[synadm](https://codeberg.org/synadm/synadm)) and the patched code works
as intended, whereas the unpatched version wants to delete users during
long vacations. 🫣
e766f325af