Scheduler assignation horizon

This module adds a 'Shipping Scheduler' section to the Inventory settings screen. There you can turn on 'Use Assignation Limit' and enter an 'Assignation Horizon' in days. When enabled, the scheduler will only try to reserve stock for stock moves whose scheduled date falls within that number of days from today; moves planned further in the future are left unreserved until they enter the horizon.

• The setting is stored globally and can be applied to all companies; the field help notes it is company-specific.
• The horizon value cannot be negative: entering a negative number raises a validation error.
• If the stored horizon value is not a valid number, the module logs a warning and falls back to 0 days.
• The behaviour is controlled through configuration parameters and is off by default.

The module extends procurement.group to filter which moves the scheduler considers for reservation, and extends res.config.settings to expose the two settings fields. It touches the Inventory settings area and the stock reservation/scheduling logic. No new menus or reports are added.