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/sqlite/migrations/tasks.json

@@ -0,0 +1,3 @@
+{
+  "tasks": []
+}