Replace raw ADO.NET implementations with EF Core LINQ queries and
ExecuteUpdate/ExecuteDelete for bulk operations. TaskRepository preserves
FlipAllRunningToFailedAsync(reason) signature and keeps raw SQL for the
atomic queue claim (UPDATE...RETURNING). GetByListAsync alias kept for
backwards compat.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>