fix(pdf-export): match DateTime.from_iso8601 three-tuple when formatting cells

DateTime.from_iso8601/1 returns {:ok, datetime, offset}, so the two-tuple clauses never matched and datetime cells fell through to the naive-parse fallback. Matching the real shape routes them through the intended DateTime path; UTC values render identically.

test/mv/membership/members_pdf_test.exs

@@ -101,6 +101,29 @@ defmodule Mv.Membership.MembersPDFTest do
       assert byte_size(pdf_binary) > 1000
     end
 
+    test "renders date column holding an ISO8601 datetime value" do
+      # Regression: a date column whose value is a full datetime string must be
+      # parsed via DateTime.from_iso8601/1 (which returns a 3-tuple) and rendered,
+      # not silently dropped.
+      export_data = %{
+        columns: [
+          %{key: "first_name", kind: :member_field, label: "Vorname"},
+          %{key: "join_date", kind: :member_field, label: "Eintritt"}
+        ],
+        rows: [
+          ["Max", "2024-01-15T14:30:00Z"]
+        ],
+        meta: %{
+          generated_at: "2024-01-15T14:30:00Z",
+          member_count: 1
+        }
+      }
+
+      assert {:ok, pdf_binary} = MembersPDF.render(export_data)
+      assert String.starts_with?(pdf_binary, "%PDF")
+      assert byte_size(pdf_binary) > 1000
+    end
+
     test "generates valid PDF with custom fields and computed fields" do
       export_data = %{
         columns: [