feat: Update RFC8414 OAuth discovery endpoint for LOLA account portability Implementation

[aaronjae22]

This PR makes a complete overview and update of the RFC8414 OAuth Authorization Server Metadata endpoint with LOLA extensions for ActivityPub account portability discovery Implementation.

Closes #228

• Created a new file (testbed/core/tests/test_lola_compliance.py) that handles the RFC8414 compliance tests
  • Delete the tests that were on testbed/core/tests/test_api.py for better separation of concerns
• Simplified the oauth_authorization_server_metadata view
• Added SECURE_PROXY_SSL_HEADER for HTTPS detection in Production (Staging inherits from it)

Features that were revised/updated

1. RFC8414-compliant metadata endpoint at /.well-known/oauth-authorization-server
2. LOLA extension - activitypub_account_portability parameter for discovery
3. HTTPS enforcement - Proper detection behind Google Cloud Run load balancer
4. CORS enabled - Cross-origin federation support
5. Comprehensive tests - 6 RFC8414 compliance tests

Production Deployment Notes

⚠️ IMPORTANT: This PR includes the fix for HTTPS URLs in production/staging.

Current behaviour: Returns http:// URLs (incorrect)

curl -sS \
  -H "Accept: application/json" \
  https://ap-testbed.dtinit.org/.well-known/oauth-authorization-server

{
   "issuer":"http://ap-testbed.dtinit.org",
   "authorization_endpoint":"http://ap-testbed.dtinit.org/oauth/authorize/",
   "token_endpoint":"http://ap-testbed.dtinit.org/oauth/token/",
   "scopes_supported":[
      "activitypub_account_portability"
   ],
   "response_types_supported":[
      "code"
   ],
   "grant_types_supported":[
      "authorization_code"
   ],
   "activitypub_account_portability":"http://ap-testbed.dtinit.org/oauth/authorize/"
}"%"

After deployment: All URLs should use https:// protocol. (correct)

The SECURE_PROXY_SSL_HEADER setting enables proper HTTPS detection behind Google Cloud Run's load balancer.

Standards Compliance

• RFC8414 (OAuth Authorization Server Metadata)
• RFC8615 (Well-Known URIs)
• LOLA v0.2 (Account Portability Specification)
• OAuth 2.0 Security Best Practices

Testing

# New LOLA compliance tests
pytest testbed/core/tests/test_lola_compliance.py -v
# Result: 6 passed 

# Existing API tests (unchanged)
pytest testbed/core/tests/test_api.py -v  
# Result: 26 passed 

Total: 32 tests passing