feat(member): add period-scoped unpaid-cycle payment-aging model

Counts each member's unpaid billing cycles that fall inside the selected
period. Cycle boundaries are member-relative — derived from each member's
fee-type interval — so "the current cycle" spans different calendar dates
per member.

lib/membership/member.ex

@@ -717,6 +717,25 @@ defmodule Mv.Membership.Member do
       constraints one_of: [:unpaid, :paid, :suspended]
     end
 
+    calculate :unpaid_cycle_count,
+              :integer,
+              expr(
+                count(membership_fee_cycles,
+                  query: [
+                    filter:
+                      expr(
+                        status == :unpaid and
+                          (is_nil(^arg(:period_from)) or cycle_end >= ^arg(:period_from)) and
+                          (is_nil(^arg(:period_to)) or cycle_end <= ^arg(:period_to))
+                      )
+                  ]
+                )
+              ) do
+      description "Count of the member's unpaid cycles whose cycle_end is within the given period"
+      argument :period_from, :date, allow_nil?: true
+      argument :period_to, :date, allow_nil?: true
+    end
+
     calculate :overdue_count, :integer do
       description "Count of unpaid cycles that have already ended (cycle_end < today)"
       # Automatically load cycles with all attributes and membership_fee_type