### Steps to reproduce

1. Install *Sales* and *Timesheets*
2. Create a service product: Invoicing Policy = *Based on Timesheets*, Create on Order = *Task*
3. Create a sale order for it (quantity 1) and confirm it
4. On the generated task, log **4.5 h on 15/06** and **3.5 h on 23/07**
5. *Create Invoice* with no timesheet period → 8 h, and post it
6. On that invoice: *Reverse* → *Partial Refund*, set the quantity to **3.5 h** and post it → 4.5 h invoiced
7. Log **1 h on 31/07** → 9 h delivered
8. *Create Invoice* again, with a **Timesheets Period of 01/06 → 31/07**

### Current behavior

The invoice bills **9 h**: the 4.5 h that were invoiced and not credited are billed a second time.

### Expected behavior

The invoice bills **4.5 h** — the quantity delivered minus the quantity invoiced.

### Cause of the issue

Posting a partial credit note clears `timesheet_invoice_id` on every timesheet the reversed invoice
had linked (`sale_timesheet/models/account_move.py`, `action_post`), because a credit note carries a
quantity and never a set of timesheets, so there is no way to tell which hours it credited.

All of those hours therefore become candidates again in `_recompute_qty_to_invoice`, which assigns
their sum to `qty_to_invoice` without comparing it to what is still due on the line.

### Fix

Timesheet links cannot express a partially invoiced timesheet, so they are used only to select the
hours a period concerns, while the quantity that may still be billed is
`qty_delivered - qty_invoiced`. The period lookup is capped by that remainder, and kept at zero or
above so that an over-invoiced line is corrected by a deliberate credit note rather than as a side
effect of invoicing a period.

### Tests

Five tests are added to `addons/sale_timesheet/tests/test_sale_timesheet.py`.

Three of them fail without the fix:

| test | without the fix |
| --- | --- |
| `test_period_invoice_does_not_rebill_refunded_invoice_hours` | `9.0 != 4.5` |
| `test_period_invoice_after_refund_is_computed_per_line` | `4.0 != 1.5` |
| `test_period_invoice_after_refund_of_an_over_invoiced_line` | `8.0 != 1.0` |

The other two cover behaviour that is not exercised today and that the fix must not break: an
over-invoiced line (which must be left out rather than credited, and must not prevent the other
lines of the order from being invoiced) and the reversed invoice's own `invoice_date`, which must not
influence the quantity billed for a period.

The full `sale_timesheet` suite passes (86 tests).