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/mv/constants.ex

@@ -38,6 +38,12 @@ defmodule Mv.Constants do
 
   @custom_date_filter_prefix "cdf_"
 
+  @payment_period_from_param "pay_from"
+
+  @payment_period_to_param "pay_to"
+
+  @payment_filter_param "pay_filter"
+
   @max_boolean_filters 50
 
   @max_mailto_bulk_recipients 50
@@ -162,6 +168,38 @@ defmodule Mv.Constants do
   """
   def custom_date_filter_prefix, do: @custom_date_filter_prefix
 
+  @doc """
+  Returns the URL parameter name for the payment-aging period lower bound
+  (ISO-8601 date; scopes both the payment column and the payment filter).
+
+  ## Examples
+
+      iex> Mv.Constants.payment_period_from_param()
+      "pay_from"
+  """
+  def payment_period_from_param, do: @payment_period_from_param
+
+  @doc """
+  Returns the URL parameter name for the payment-aging period upper bound.
+
+  ## Examples
+
+      iex> Mv.Constants.payment_period_to_param()
+      "pay_to"
+  """
+  def payment_period_to_param, do: @payment_period_to_param
+
+  @doc """
+  Returns the URL parameter name for the payment-count filter
+  (`fully_paid` | `unpaid_1` | `unpaid_2` | `unpaid_3`).
+
+  ## Examples
+
+      iex> Mv.Constants.payment_filter_param()
+      "pay_filter"
+  """
+  def payment_filter_param, do: @payment_filter_param
+
   @doc """
   Returns the maximum number of boolean custom field filters allowed per request.