feat: implement full CRUD for membership fee types with settings UI

- Add interval immutability and deletion prevention validations
- Add settings validation for default_membership_fee_type_id
- Create MembershipFeeSettingsLive for admin UI with form handling
- Add comprehensive test coverage (unit, integration, settings)

test/membership/membership_fee_settings_test.exs

@@ -0,0 +1,99 @@
+defmodule Mv.Membership.MembershipFeeSettingsTest do
+  @moduledoc """
+  Tests for membership fee settings in the Settings resource.
+  """
+  use Mv.DataCase, async: true
+
+  alias Mv.Membership.Setting
+  alias Mv.MembershipFees.MembershipFeeType
+
+  describe "membership fee settings" do
+    test "default values are correct" do
+      {:ok, settings} = Mv.Membership.get_settings()
+      assert settings.include_joining_cycle == true
+    end
+
+    test "settings can be read" do
+      {:ok, settings} = Mv.Membership.get_settings()
+      assert %Setting{} = settings
+    end
+
+    test "settings can be written via update_membership_fee_settings" do
+      {:ok, settings} = Mv.Membership.get_settings()
+
+      {:ok, updated} =
+        settings
+        |> Ash.Changeset.for_update(:update_membership_fee_settings, %{
+          include_joining_cycle: false
+        })
+        |> Ash.update()
+
+      assert updated.include_joining_cycle == false
+    end
+
+    test "default_membership_fee_type_id can be nil (optional)" do
+      {:ok, settings} = Mv.Membership.get_settings()
+
+      {:ok, updated} =
+        settings
+        |> Ash.Changeset.for_update(:update_membership_fee_settings, %{
+          default_membership_fee_type_id: nil
+        })
+        |> Ash.update()
+
+      assert updated.default_membership_fee_type_id == nil
+    end
+
+    test "default_membership_fee_type_id validation: must exist if set" do
+      {:ok, settings} = Mv.Membership.get_settings()
+
+      # Create a valid fee type
+      {:ok, fee_type} =
+        Ash.create(MembershipFeeType, %{
+          name: "Test Fee Type #{System.unique_integer([:positive])}",
+          amount: Decimal.new("100.00"),
+          interval: :yearly
+        })
+
+      # Setting a valid fee type should work
+      {:ok, updated} =
+        settings
+        |> Ash.Changeset.for_update(:update_membership_fee_settings, %{
+          default_membership_fee_type_id: fee_type.id
+        })
+        |> Ash.update()
+
+      assert updated.default_membership_fee_type_id == fee_type.id
+    end
+
+    test "default_membership_fee_type_id validation: fails if not found" do
+      {:ok, settings} = Mv.Membership.get_settings()
+
+      # Use a non-existent UUID
+      fake_uuid = Ecto.UUID.generate()
+
+      assert {:error, error} =
+               settings
+               |> Ash.Changeset.for_update(:update_membership_fee_settings, %{
+                 default_membership_fee_type_id: fake_uuid
+               })
+               |> Ash.update()
+
+      assert error_on_field?(error, :default_membership_fee_type_id)
+    end
+  end
+
+  # Helper to check if an error occurred on a specific field
+  defp error_on_field?(%Ash.Error.Invalid{} = error, field) do
+    Enum.any?(error.errors, fn e ->
+      case e do
+        %{field: ^field} -> true
+        %{fields: fields} when is_list(fields) -> field in fields
+        _ -> false
+      end
+    end)
+  end
+
+  defp error_on_field?(_, _), do: false
+end
+