last
Observable flow of the last delta sync timestamp for User archive.
Emits ISO 8601 formatted datetime string (e.g., "2025-01-15T10:30:00Z") when available. Does not emit if no delta sync has been performed yet - consumers should handle the absence of emissions with appropriate fallback logic.
Updates automatically after each successful delta sync operation.
Return
Flow of formatted last sync time (only emits when timestamp exists)