feat: add multi-database support with generic environment variables

[DevStarlight]

Description

This PR refactors the database configuration system to support multiple database engines with a more flexible and generic approach. The changes replace PostgreSQL-specific environment variables (POSTGRES_) with generic database variables (DB_) and add support for MySQL alongside the existing PostgreSQL and SQLite options.

Key Changes:

• Replaced POSTGRES_HOST, POSTGRES_PORT, POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_DB with generic DB_HOST, DB_PORT, DB_USER, DB_PASSWORD, DB_NAME
• Added DB_TYPE environment variable to specify database engine (pg for PostgreSQL, mysql2 for MySQL)
• Added MySQL configuration support with appropriate charset and timezone settings
• Configured pg and mysql2 as optional dependencies to keep bundle size minimal
• Maintained backward compatibility with SQLite as the default fallback option

This improves the server infrastructure flexibility allowing developers to choose their preferred database without being locked into PostgreSQL.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Component enhancement

How Has This Been Tested?

Please describe your tests:

• Component functionality tests
• Integration tests with other Hyperfy components
• World integration testing
• Cross-browser testing

Test Configuration:

• Hyperfy Version: 0.13.0
• Test world setup: Local development environment with SQLite fallback
• Browser(s): Chromium
• Node.js version: v22.13.1

Checklist:

• My code follows Hyperfy's component architecture
• I have performed a self-review
• I have documented the component's usage
• I have tested the component in multiple scenarios
• My changes maintain compatibility with existing worlds
• I have updated the component documentation if needed

[DevStarlight]

Should I include any specific doc somewhere? If so, please tell me where should be the best place

[DevStarlight]

closing PR since I combined this one and #99 in #100