Main repo integration for SCIM

[khvn26]

Scope

Wire the SCIM module from flagsmith-private into the main repo:

• Conditional loading in settings/common.py: check for the SCIM module (e.g. importlib.util.find_spec), add to INSTALLED_APPS, configure DJANGO_SCIM_ settings pointing to adapters and middleware in flagsmith-private.
• Mount SCIM URLs at /api/v1/scim/v2/ in the main URL conf, conditionally.
• Mount SCIM configuration management URLs under /api/v1/organisations/{org_id}/scim/, conditionally.

Acceptance criteria

• When the SCIM module is not installed, no SCIM URLs are registered and no SCIM app appears in INSTALLED_APPS.
• When installed, GET /api/v1/scim/v2/ServiceProviderConfig returns a valid SCIM ServiceProviderConfig response.
• GET /api/v1/scim/v2/Schemas and GET /api/v1/scim/v2/ResourceTypes return valid responses.
• Requests without a valid bearer token receive 401.
• Requests from a non-Enterprise org receive 403.