Admin changelist N+1 query causes Gunicorn worker timeout on /admin/sensors/sensorlocation/

Labels: bug, performance, admin
Linked Sentry: SENSORSAFRICA-API-13Z — https://code-for-africa.sentry.io/issues/7569887953/
Environment: production

`/admin/sensors/sensorlocation/` triggers a per-row N+1 query pattern. A separate `COUNT(*) FROM sensors_sensordatavalue` plus a separate node lookup for each row in the changelist. With default `list_per_page` (~100), this fires 100+ sequential queries per page load, exceeding Gunicorn's worker timeout, which triggers a SIGABRT and `SystemExit: 1`.

## Root cause
`SensorLocationAdmin` (or a related inline) lacks `select_related`/`prefetch_related`/`annotate` in `get_queryset()`. Likely a `list_display` callable or `__str__` touching `sensordata_set.count()` per row.