fix: implement fail-closed behavior in ValidateSameInterval

Change validation to fail closed instead of fail open when types cannot
be loaded. This prevents inconsistent data states and provides clearer
error messages to users.

lib/membership_fees/changes/validate_same_interval.ex

@@ -62,10 +62,10 @@ defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
           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
+      {:error, reason} ->
+        # Fail closed: If we can't load the types, reject the change
+        # This prevents inconsistent data states
+        add_type_validation_error(changeset, reason)
     end
   end
 
@@ -114,6 +114,17 @@ defmodule Mv.MembershipFees.Changes.ValidateSameInterval do
     )
   end
 
+  # Add validation error when types cannot be loaded
+  defp add_type_validation_error(changeset, reason) do
+    message = "Could not validate membership fee type intervals: type not found"
+
+    Ash.Changeset.add_error(
+      changeset,
+      field: :membership_fee_type_id,
+      message: message
+    )
+  end
+
   # Format interval atom to human-readable string
   defp format_interval(:monthly), do: "monthly"
   defp format_interval(:quarterly), do: "quarterly"