feat: enforce durable queue resource limits and backpressure

internal/fault/fault.go

@@ -25,6 +25,7 @@ var (
 	Conflict            = New("STATE_CONFLICT", "resource state does not allow this operation")
 	IdempotencyConflict = New("IDEMPOTENCY_CONFLICT", "idempotency key was used with different content")
 	Unavailable         = New("UNAVAILABLE", "service temporarily unavailable; retry using the original operation identity")
+	QueueFull           = New("QUEUE_FULL", "queue unfinished task limit reached; retry later using the original identity")
 )
 
 func Invalid(message string) error { return New("INVALID_ARGUMENT", message) }