[3] Implement license in django back

Connect license with limits on the number of cameras.

Look for existing license and camera limit models in the codebase, likely in a `licenses` or `billing` app. Examine how cameras are currently tracked and where access control is enforced. The feature needs to connect these systems, possibly adding validation in camera creation endpoints or a middleware.

索引モデルが issue の本文から書いたものです。