feat(member): resolve custom-field filters and sorting in PostgreSQL

Push the boolean and date custom-field predicates and custom-field
sorting down to JSONB expressions so the overview stops classifying
custom-field values in memory. A GIN index on custom_field_values.value
keeps the boolean membership predicates index-served.

priv/repo/migrations/20260630170637_add_custom_field_values_value_gin_index.exs

@@ -0,0 +1,21 @@
+defmodule Mv.Repo.Migrations.AddCustomFieldValuesValueGinIndex do
+  @moduledoc """
+  Adds a GIN index on `custom_field_values.value` (JSONB) to serve the boolean
+  custom-field containment predicates used by the member overview.
+  """
+
+  use Ecto.Migration
+
+  def up do
+    create index(:custom_field_values, ["value"],
+             name: "custom_field_values_value_gin_index",
+             using: "gin"
+           )
+  end
+
+  def down do
+    drop_if_exists index(:custom_field_values, ["value"],
+                     name: "custom_field_values_value_gin_index"
+                   )
+  end
+end