Changes

Connect to existing PostgreSQL 18 server

Remove embedded PostgreSQL and connect to external postgres-postgres container:
- Remove postgresql service from docker-compose.yml
- Update environment variables for external PostgreSQL connection
- Add database initialization stage in Jenkins pipeline
- Update all documentation (README.md, CLAUDE.md)
- Add extra_hosts for container-to-container networking
- Update backup and verification stages for external database

Database Architecture:
- Authentik now uses existing PostgreSQL 18 deployment
- PostgreSQL managed separately in postgresql/ repository
- Automated database initialization via Jenkins
- Shared PostgreSQL reduces resource usage and centralizes database management