[feature] Invalidate VPN cache when Organization Configuration variable changes

[pandafy]

Is your feature request related to a problem? Please describe.
When an Organization Configuration variable changes in OpenWISP Controller, the VPN cache is not automatically invalidated. This can lead to outdated VPN configurations being used, causing inconsistency and connectivity issues.

Describe the solution you'd like
Implement a mechanism to automatically invalidate the VPN cache whenever an Organization Configuration variable is changed, ensuring that VPN always use the latest configuration.

Related to

• [bug] Device config status does not update to "modified" when organization variable is changed #1070
• [feature] Generic solution for invalidating cache on related field changes #1095

[lakshay909]

Hi @pandafy, I'd like to work on this issue — thank you for maintaining such a well-structured project!
I’m a student developer experienced in Django, REST APIs, and open-source backend systems, and I’ve previously contributed to projects focused on improving API logic and data synchronization. For this issue, my plan is to review the related model and signal flow, reproduce the described behavior locally, and then implement a clean fix ensuring consistency with the existing codebase and test coverage. I’ll also add or update relevant unit tests to confirm the change works as expected. I can open a PR within 3–5 days after assignment.

Kindly assign this issue to me — I’d love to contribute to OpenWISP and ensure a reliable fix.