[IMP] subscription_oca: track invoice periods on move lines

[alvaro-domatix]

Problem

Invoice lines created from a subscription do not say which billing period they cover and have no link back to the originating subscription. Accounting cannot tell at a glance which month an invoice line bills, and there is no easy way to query revenue by subscription.

Solution

Add three new fields on account.move.line:

• subscription_id: link to the originating sale.subscription (indexed, ondelete='set null').
• subscription_period_start / subscription_period_end: dates of the period billed on this line.

A new helper sale.subscription._get_invoice_period() returns the (period_start, period_end) tuple for the current recurring cycle: start = recurring_next_date, end = start + recurrence - 1 day.

sale.subscription.line._prepare_account_move_line now accepts optional period_start and period_end kwargs. When they are provided, both fields are populated and the line description is enriched with the period formatted in the partner's language (e.g. "Product (01/05/2026 - 31/05/2026)").

sale.subscription.create_invoice computes the period once and passes it to every line, so manual_invoice, generate_invoice and the cron all benefit without further changes.

How to test

Run:

odoo-bin -d <db> --init=subscription_oca --stop-after-init
odoo-bin -d <db> -u subscription_oca --test-enable --test-tags=/subscription_oca --stop-after-init

The new tests/test_subscription_invoice_period.py covers:

• _get_invoice_period() returns the expected (start, end) pair for a monthly template.
• After create_invoice(), every generated move line has the matching subscription_id, subscription_period_start and subscription_period_end.
• Move line descriptions contain the period dates and the " - " separator.