feat(ci): add license verification workflow

[ncarenton]

Description

Add a CI workflow to verify that all Python dependencies comply with an approved license allowlist.

Source of Inspiration

The OpenCTI main repository already has such a workflow:
https://github.com/OpenCTI-Platform/opencti/blob/master/.github/workflows/ci-license-check.yml

It uses license-checker-rseidelsohn (Node.js) with an allowlist of approved licenses. We need the Python equivalent for this repository.

Suggested Implementation

• Tooling: pip-licenses with --allow-only flag (fails if any dependency uses an unlisted license)
• Trigger: workflow_call (reusable workflow)
• Scope: Verify all Python dependencies declared in requirements files

Suggested Allowlist

Same as OpenCTI plus Python-specific additions:

• 0BSD, Apache-2.0, BlueOak-1.0.0, BSD-2-Clause, BSD-3-Clause
• CC-BY-3.0, CC-BY-4.0, CC0-1.0, Hippocratic-2.1
• ISC, LGPL-3.0, MIT, OFL-1.1
• Python-2.0, PSF-2.0, MPL-2.0
• Unlicense