MembersPDF test: async false and try/after to avoid flakiness

Tests that remove the template file run sequentially; restore
template in after block so it is restored even when assertion fails.

test/mv/membership/members_pdf_test.exs

@@ -8,7 +8,9 @@ defmodule Mv.Membership.MembersPDFTest do
   - Generates valid PDF binary (starts with "%PDF")
   - Handles errors gracefully
   """
-  use ExUnit.Case, async: true
+  # async: false so tests that manipulate the template file (e.g. "returns error when template file is missing")
+  # do not run in parallel and cannot leave the template removed on failure
+  use ExUnit.Case, async: false
 
   alias Mv.Config
   alias Mv.Membership.MembersPDF
@@ -162,13 +164,13 @@ defmodule Mv.Membership.MembersPDFTest do
     end
 
     test "returns error when template file is missing" do
-      # Temporarily rename template to simulate missing file
       template_path =
         Path.join(Application.app_dir(:mv, "priv"), "pdf_templates/members_export.typ")
 
       original_content = File.read!(template_path)
       File.rm(template_path)
 
+      try do
         export_data = %{
           columns: [%{key: "first_name", kind: :member_field, label: "Vorname"}],
           rows: [["Max"]],
@@ -178,10 +180,10 @@ defmodule Mv.Membership.MembersPDFTest do
         result = MembersPDF.render(export_data)
 
         assert {:error, {:template_not_found, _reason}} = result
-
+      after
-      # Restore template
         File.write!(template_path, original_content)
       end
+    end
 
     test "handles empty rows gracefully" do
       export_data = %{