Default unknown free_busy values to busy

[mmonterroca]

Summary

Default missing or unknown free_busy values to busy when saving events through the database driver.

This prevents iTIP imports/updates from failing when an incoming ICS does not provide a recognized free/busy value.

Observed error

In production, an iTIP update from a Microsoft Teams / Outlook invitation failed with:

DB Error: [1048] Column 'free_busy' cannot be null
POST /?_task=calendar&_framed=1&_action=mailimportitip

The failing SQL attempted to update an existing event with:

`free_busy` = NULL

The MySQL schema defines the column as:

`free_busy` tinyint(1) NOT NULL DEFAULT '0'

Cause

_save_preprocess() maps missing or unrecognized free_busy values to null:

$event['free_busy'] = $this->free_busy_map[$free_busy] ?? null;

That conflicts with the database schema during updates.

Fix

Use busy as the default fallback:

$event['free_busy'] = $this->free_busy_map[$free_busy] ?? $this->free_busy_map['busy'];

For meeting invitations, defaulting to busy is safer than failing the import/update and matches normal calendar behavior when no transparency value is present.

Testing

• Applied the same one-line patch to a Roundcube 1.6.x / kolab/calendar 3.6.1 production install.
• Validated syntax there with php -l drivers/database/database_driver.php.
• Re-ran previously failing Microsoft Teams iTIP updates; they imported/updated successfully.

Note: I could not open a separate issue because issues are disabled on this repository.