add support for SQLite as an alternative database backend

Implement SQLite support using the pure Go `modernc.org/sqlite` driver and update the migration system to handle driver-specific schemas. Users can now choose between MySQL and SQLite by setting the `DB_DRIVER` environment variable.

internal/database/schema/mysql/migrations/2_users.sql

@@ -0,0 +1 @@
+ALTER TABLE user ADD COLUMN enabled BOOLEAN NOT NULL DEFAULT TRUE AFTER role;