Build BufdirSchemaConfigRepository Dart class

Implement the Riverpod cache as an AsyncNotifier keyed by org_id using a family modifier: schemaConfigProvider.family. On upsert, call ref.invalidate(schemaConfigProvider(orgId)) to force a fresh fetch on next access. For detectSchemaDrift, compare List using column key as the unique identifier — two columns with the same position but different keys are a rename candidate. Return a SchemaDriftResult freezed class with hasDrift (bool), addedKeys (List), removedKeys (List), and renamedPairs (List<(String from, String to)>).

Keep this function in a separate schemaDriftUtils.dart file for testability. The upsert conflict check (same version, different columns) should be performed by fetching the existing row before upsert and comparing — do not rely on a database constraint alone since Supabase upsert silently overwrites by default.

Unit tests with mocktail: (1) getActiveSchema returns null when Supabase returns empty list; (2) getActiveSchema returns the highest-version row when multiple rows exist; (3) upsertSchemaVersion throws SchemaUpsertForbiddenException when JWT role is not super_admin; (4) upsertSchemaVersion calls Supabase upsert with correct payload; (5) detectSchemaDrift returns empty SchemaDriftResult for identical schemas; (6) detectSchemaDrift correctly identifies one added column; (7) detectSchemaDrift correctly identifies one removed column; (8) cache returns cached value on second call without triggering Supabase fetch; (9) cache invalidates after upsertSchemaVersion. Integration tests: verify that getActiveSchema fetches the live active schema and that upsertSchemaVersion succeeds for a super_admin token. Minimum 85% line coverage on the repository and drift detection logic.