Add settled_at to zaps, scope by settlement status

app/models/zap.rb

@@ -1,6 +1,9 @@
 class Zap < ApplicationRecord
   belongs_to :user
 
+  scope :settled, -> { where.not(settled_at: nil) }
+  scope :unpaid, -> { where(settled_at: nil) }
+
   def request_event
     nostr_event_from_hash(request)
   end