fix: Compute user last seen timestamp from last seen devices (#18948)

## 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. 🫣

changelog.d/18948.bugfix

@@ -0,0 +1 @@
+Compute a user's last seen timestamp from their devices' last seen timestamps instead of IPs, because the latter are automatically cleared according to `user_ips_max_age`.

synapse/storage/databases/main/__init__.py

@@ -299,10 +299,14 @@ class DataStore(
                 FROM users as u
                 LEFT JOIN profiles AS p ON u.name = p.full_user_id
                 LEFT JOIN erased_users AS eu ON u.name = eu.user_id
+                LEFT JOIN (
+                    SELECT user_id, MAX(last_seen) AS last_seen_ts
+                    FROM devices GROUP BY user_id
+                ) lsd ON u.name = lsd.user_id
                 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
+                ) lsi ON u.name = lsi.user_id
                 {where_clause}
                 """
             sql = "SELECT COUNT(*) as total_users " + sql_base
@@ -312,7 +316,8 @@ class DataStore(
             sql = f"""
                 SELECT name, user_type, is_guest, admin, deactivated, shadow_banned,
                 displayname, avatar_url, creation_ts * 1000 as creation_ts, approved,
-                eu.user_id is not null as erased, last_seen_ts, locked
+                eu.user_id is not null as erased,
+                COALESCE(lsd.last_seen_ts, lsi.last_seen_ts) as last_seen_ts, locked
                 {sql_base}
                 ORDER BY {order_by_column} {order}, u.name ASC
                 LIMIT ? OFFSET ?