fix: improve OIDC implementation and documentation

[ervcz]

Summary

• Refactor authentication system to use authorization model for OIDC
• Add comprehensive OIDC test infrastructure with E2E tests
• Improve time execution of cache testing by 87.5%
• Add thorough documentation and migration guides

Key Changes

• OIDC Authentication: Complete refactor from session-based to stateless JWT token validation
• Test Infrastructure: New Docker compose setup for OIDC testing with Keycloak
• API Optimization: Improved cache test execution time from 60s+ to 100ms
• Enhanced Documentation: Added migration guide and architecture decision records
• Security Improvements: Deprecated insecure password grant flow

Test Plan

• [ ] Backend unit tests pass
• [ ] OIDC E2E tests with Keycloak integration
• [ ] API performance tests verify cache optimization
• [ ] Frontend integration tests
• [ ] Documentation review