[18.0] [FIX] subscription_oca: add migration script for subscription_template_id field on product.template model

[rrebollo]

Context

We discovered this issue during a customer migration from v14 to v18: the subscription_template_id field value for product templates is lost after upgrading to v18.

Root Cause

#1060 made subscription_template_id a company-dependent field on product.template, but this change was never forward-ported beyond its target version (v14).

In Odoo 18.0, company-dependent fields are stored in a JSONB column instead of ir_property records. Since the forward-port never happened, the automatic migration from ir_property to JSONB did not occur, leaving the field empty after upgrade.

Solution

This pull request adds a pre-migration script for Odoo 18.0 that backfills product_template.subscription_template_id from legacy ir_property records, ensuring existing data is preserved during the upgrade.

Scope

This fix applies in a way that only instances affected by the company-dependent implementation of the subscription_template_id field will be targeted. Nothing changes or backfires if you're not affected.

Migration and Data Integrity

• Added pre-migration.py to backfill product_template.subscription_template_id from legacy ir_property records
• Documented the root cause and provided context about the storage change for company-dependent fields

[luisDIXMIT]

Code review, LGTM!