Implement core Admin Repository query methods

Structure AdminRepository as a plain Dart class (not a Riverpod StateNotifier) provided via Provider. Inject AdminRlsGuard via constructor. Each method should follow the pattern: final client = await _guard.guardedClient; final response = await client.from('table').select('col1,col2,...').execute(); return response.data.map((row) => DomainModel.fromJson(row)).toList(). Define AdminRepositoryException as a typed exception carrying operationName, message, and optional statusCode.

For pagination, add optional AdminQueryPage page parameter (with offset and limit) to each method signature now — even if the UI only uses the first page initially — to avoid breaking changes later. Align column names precisely with the Supabase schema established in task-003.

Unit tests (flutter_test with mocked AdminRlsGuard and mocked SupabaseClient): (1) Each method calls guardedClient exactly once per invocation. (2) Each method correctly maps raw Supabase JSON response to typed domain models. (3) Empty database response returns empty list. (4) Supabase error is wrapped in AdminRepositoryException with correct method name.

(5) Optional filters are applied correctly (verify query builder receives correct .eq()/.gte() calls). Integration tests against local Supabase: (6) org_admin user only sees users/activities within their org subtree. (7) super_admin sees cross-org data. (8) Date range filter on getActivitiesInScope returns only records in range.