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)

lib/membership/setting.ex

@@ -141,6 +141,21 @@ defmodule Mv.Membership.Setting do
                end
              end,
              on: [:create, :update]
+
+    # Validate default_membership_fee_type_id exists if set
+    validate fn changeset, _context ->
+      fee_type_id = Ash.Changeset.get_attribute(changeset, :default_membership_fee_type_id)
+
+      if fee_type_id do
+        case Ash.get(Mv.MembershipFees.MembershipFeeType, fee_type_id) do
+          {:ok, _} -> :ok
+          {:error, _} ->
+            {:error, field: :default_membership_fee_type_id, message: "Membership fee type not found"}
+        end
+      else
+        :ok  # Optional, can be nil
+      end
+    end, on: [:create, :update]
   end
 
   attributes do