refactor: reduce nesting depth and improve code readability

- Replace Enum.map |> Enum.join with Enum.map_join for efficiency
- Extract helper functions to reduce nesting depth from 4 to 2
- Rename is_current_cycle? to current_cycle? following Elixir conventions

lib/membership_fees/changes/validate_same_interval.ex

@@ -37,29 +37,35 @@ defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
     current_type_id = get_current_type_id(changeset)
     new_type_id = get_new_type_id(changeset)
 
-    # If no current type, allow any change (first assignment)
-    if is_nil(current_type_id) do
-      changeset
-    else
-      # If new type is nil, that's allowed (removing type)
-      if is_nil(new_type_id) do
+    cond do
+      # If no current type, allow any change (first assignment)
+      is_nil(current_type_id) ->
         changeset
-      else
-        # Both types exist - validate intervals match
-        case get_intervals(current_type_id, new_type_id) do
-          {:ok, current_interval, new_interval} ->
-            if current_interval == new_interval do
-              changeset
-            else
-              add_interval_mismatch_error(changeset, current_interval, new_interval)
-            end
 
-          {:error, _reason} ->
-            # If we can't load the types, allow the change (fail open)
-            # The database constraint will catch invalid foreign keys
-            changeset
+      # If new type is nil, that's allowed (removing type)
+      is_nil(new_type_id) ->
+        changeset
+
+      # Both types exist - validate intervals match
+      true ->
+        validate_intervals_match(changeset, current_type_id, new_type_id)
+    end
+  end
+
+  # Validates that intervals match when both types exist
+  defp validate_intervals_match(changeset, current_type_id, new_type_id) do
+    case get_intervals(current_type_id, new_type_id) do
+      {:ok, current_interval, new_interval} ->
+        if current_interval == new_interval do
+          changeset
+        else
+          add_interval_mismatch_error(changeset, current_interval, new_interval)
         end
-      end
+
+      {:error, _reason} ->
+        # If we can't load the types, allow the change (fail open)
+        # The database constraint will catch invalid foreign keys
+        changeset
     end
   end