**Patch description:**
- This commit addresses a perf issue in the `_compute_previous_order` compute of the `pos.order` model, which is particularly noticeable during db upgrade from `v17.0` to `v18.0` because field `previous_order_id` got introduced in `v18.0` or when installing the `l10n_fr_pos_cert` module on an existing db with many POS orders in `v18.0`.

- In both cases, the compute method is triggered for all `pos.order` records to assign the `previous_order_id`, causing significant delay due to per-record lookups.

**Example from a real DB with ~113k orders:**
```sql
apan_2840187=> select count(*) from pos_order;
 count
--------
 113342
(1 row)
```

**Observation:**
- Before the fix (compute took nearly 50 minutes):
```py
2025-05-29 06:31:12,587 28 INFO apan_2840187_18.0 odoo.modules.registry: module l10n_fr_pos_cert: creating or updating database tables
2025-05-29 06:31:12,799 28 INFO apan_2840187_18.0 odoo.models: Prepare computation of pos.order.previous_order_id
2025-05-29 07:17:37,855 28 INFO apan_2840187_18.0 odoo.modules.loading: loading l10n_fr_pos_cert/views/pos_views.xml
```

- After the fix (completed within 1 min):
```py
2025-05-30 06:56:59,510 24 INFO apan_2840187_18.0 odoo.modules.registry: module l10n_fr_pos_cert: creating or updating database tables
2025-05-30 06:56:59,584 24 INFO apan_2840187_18.0 odoo.models: Prepare computation of pos.order.previous_order_id
2025-05-30 06:57:12,531 24 INFO apan_2840187_18.0 odoo.modules.loading: loading l10n_fr_pos_cert/views/pos_views.xml
```

**Note:** The issue reported in the OPW was not originally about perf, but it was clearly exposed during the upgrade of the mentioned db.

opw-4812874
upg-2840187


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