feat(backend): add SQLite to PostgreSQL migration support

[shepherd-001]

Closes #531

Summary

This PR adds an MVP-ready migration path from SQLite to PostgreSQL for the Vaultix backend.

Changes

• Added database driver selection through DATABASE_DRIVER.
• Added PostgreSQL connection configuration with connection pooling.
• Updated TypeORM migrations and entities for PostgreSQL compatibility.
• Handled JSON and date/time differences between SQLite and PostgreSQL.
• Added SQLite-to-PostgreSQL data migration support.
• Updated database health checks to report the configured driver.
• Added SQLite and PostgreSQL database coverage to CI.
• Updated environment, Docker, and deployment documentation for PostgreSQL.

Validation

• Verified existing migrations and entities against PostgreSQL.
• Confirmed SQLite remains supported for local development.
• Tested database initialization, migration, and data transfer workflows.
• Verified PostgreSQL configuration and health-check behavior.

Scope

This is intentionally focused on the required database migration without introducing unnecessary architectural changes.

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