[19.0][MIG] hr_expense_invoice: Migration to 19.0

[dnplkndll]

Port of `hr_expense_invoice` from 18.0 to 19.0 (migration guide).

Non-mechanical adaptations worth flagging

• Hook target moved from `hr.expense.sheet.action_sheet_move_post` to `hr.expense.action_post()`. 19.0 core's `action_post` (`addons/hr_expense/models/hr_expense.py:1169`) auto-creates a receipt (own_account) or payment (company_account) per expense. Bill-linked expenses bypass that and route through the AP transfer-entry flow.
• `account.move.expense_ids` renamed to `invoice_expense_ids` to avoid clash with 19.0 core's same-named One2many (which is the receipt-flow inverse on `account_move_id`, not the bill-link inverse on `invoice_id`). The picker domain on `hr.expense.invoice_id` now uses `invoice_expense_ids`.
• `account.move.line.reconcile` cascade simplified. 18.0 traversed expense → sheet → set_to_paid. In 19.0, `hr.expense.state` is a computed mirror of `approval_state` + move/payment state; invalidating `amount_residual` after the AP-transfer line is reconciled is enough to recompute the cascade.
• Sheet-only view block dropped (`hr_expense_sheet_form_view_inherit_sale_expense`) — `hr.expense.sheet` is gone. The `invoice_id` m2o on the expense form already exposes the "Create" context for new bills, replacing the per-line "Create Vendor Bill" button.
• Bill-linked company_account path writes `approval_state='approved'` directly. Core's `_create_company_paid_moves` would otherwise duplicate the bill's obligation.

19.0 relevance

Two flows the module preserves that 19.0 core does not absorb:

1. Pre-existing bill link — when a supplier sends an invoice to accounts (separately from the expense system), this module lets an employee file an expense that references the existing bill. Core's auto-receipt flow has no concept of "the bill already exists, just attach it."
2. AP transfer entry for employee-paid bill-linked expenses — debit the bill's vendor AP, credit the employee's payable. Pays the employee through the existing bill-payment cycle instead of generating a parallel receipt.

UX with sheets removed: the expense form's `invoice_id` m2o is the entry point. Smart-button on `account.move` shows linked expense. Batch "create bill from N expenses" UX from the 18.0 sheet button is dropped — the 19.0 equivalent is selecting multiple expenses in the list and using core's per-expense `action_expense_create_invoice` (or a list-view action — out of scope for this slim MIG). Happy to add it if reviewers ask.

CI may surface tweaks needed on view xpaths (19.0's `hr.expense` form structure shifted around `product_id`/`quantity`); will iterate.