Implement complete PostgreSQL support as an alternative to MongoDB,
allowing users to select their preferred database backend via
SHELLHUB_DATABASE environment variable.

Infrastructure:
- Add PostgreSQL 18.0 service in docker-compose.postgres.yml
- Configure PostgreSQL environment variables (username, password,
database)
- Update bin/utils to recognize 'postgres' as valid database option
- Add PostgreSQL healthcheck configuration

Store Implementation:
- Create pg store package with bun ORM and pgx driver
- Implement all CRUD operations for all store interfaces
- Add entity package with complete model-to-database mappings
- Create 10 database migrations covering all tables and relations
- Implement query system with pagination, sorting, and filtering
- Add internal filter parser for complex queries (contains, eq, bool,
gt, ne)
- Implement SQL error mapping and handling utilities
- Create transaction support with bun

Entities and Relations:
- Namespaces, Users, Memberships
- Devices with status tracking
- API Keys, Public Keys, Private Keys
- Tags with many-to-many relations (device-tags, publickey-tags)
- Sessions and Tunnels

Testing Infrastructure:
- Add testcontainers-based integration tests
- Create YAML fixture system for test data
- Implement comprehensive unit tests for all store operations
- Add database test utilities and helpers

Integration:
- Update api/server.go to support database selection
- Update cli/main.go to support database selection
- Add bun, pgx, and testcontainers dependencies
- Maintain backward compatibility with existing MongoDB implementation