add explicit permissions and workflow_call to scheduled-checks

[miguemagicdev]

Description

This PR updates scheduled-checks.yml to resolve a permission denied error that caused the workflow to fail after an 8-hour timeout.

The issue: The workflow attempted to upload security scan results but encountered the following error:

"The workflow is requesting 'security-events: write', but is only allowed 'security-events: none'"

Changes

• Added permissions block: Explicitly granted security-events: write (required for dispatching the status checks) and contents: read (required to checkout the code).
• Added workflow_call trigger: Enabled the workflow to be triggered manually from other workflows. This allows us to verify the permission fix immediately without waiting for the next 8-hour scheduled interval.