Issue
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Opening a PoS sends every partner cached on the device to `get_all_total_due`. On a production database, six of those calls within 30 minutes ran 7061 queries, the worst one 4837 queries in 46.7 seconds, keeping a worker busy for the whole duration and slowing down the entire system.

Cause
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`get_all_total_due` loops over the recordset and calls `get_total_due` once per partner. Each iteration runs its own `pos.order` search for the open pay later orders and re-reads `point_of_sale.group_pos_user`, hence exactly two queries per partner. Every other value the method needs (`total_due`, `pos_orders_amount_due`, `invoices_amount_due` and the `_load_pos_data_read` payload) is already computed for the whole recordset at once by the ORM prefetching, so only those two remained unbatched.

That search is also the expensive one: `commercial_partner_id` is not indexed, so each of them scans the whole set of orders in state 'paid'.

Fix
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Move the computation to `_get_total_due_data`, which queries the orders, the pay later payments and the partners data once for the whole recordset. `get_total_due` and `get_all_total_due` keep their signature and their return value, and both delegate to it.

Measured on a database with 300k pos orders, for 1000 partners: 2193 queries / 3.98s before, 16 queries / 0.27s after.

opw-6530561

Forward-Port-Of: odoo/enterprise#130334