Add filter prefix constants and shared FilterParams module

- Mv.Constants: group_filter_prefix/0, fee_type_filter_prefix/0
- MvWeb.MemberLive.Index.FilterParams: parse_in_not_in_value/1 for URL param parsing

lib/mv_web/live/member_live/index/filter_params.ex

@@ -0,0 +1,22 @@
+defmodule MvWeb.MemberLive.Index.FilterParams do
+  @moduledoc """
+  Shared parsing helpers for member list filter URL/params (in/not_in style).
+  Used by MemberLive.Index and MemberFilterComponent to avoid duplication and recursion bugs.
+  """
+  @doc """
+  Parses a value for group or fee-type filter params.
+  Returns `:in`, `:not_in`, or `nil`. Handles trimmed strings; no recursion.
+  """
+  def parse_in_not_in_value("in"), do: :in
+  def parse_in_not_in_value("not_in"), do: :not_in
+
+  def parse_in_not_in_value(val) when is_binary(val) do
+    case String.trim(val) do
+      "in" -> :in
+      "not_in" -> :not_in
+      _ -> nil
+    end
+  end
+
+  def parse_in_not_in_value(_), do: nil
+end