Keep dinner names frozen in Supply archive posts

Freezing a Supply run now records each group's dinner name in
shopping_groups.frozen_dinner_name, in the same transaction as the
freeze. A frozen list and its Supply archive post read that name, so
renaming a dinner after its week is frozen no longer rewrites the
archive; a current list still shows its dinners' names live.
Migration 0018 adds the column and fills it for lists frozen before it.

Task: 22f80eb6-cdee-4377-80ea-9b28f15a866f

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/shopping/archive_test.go

@@ -422,7 +422,7 @@ func TestArchiveGroups(t *testing.T) {
 	if _, err := e.st.Writer.Exec(`UPDATE shopping_lists SET title = 'Chili, Noodles & Co' WHERE week_start = '2026-12-28'`); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := e.st.Writer.Exec(`UPDATE shopping_lists SET status = 'archived', frozen_at = '2027-01-03T11:05:00.000Z' WHERE week_start < '2027-01-04'`); err != nil {
+	if _, err := e.st.Writer.Exec(`UPDATE shopping_lists SET status = 'archived', frozen_at = '2027-01-03T11:05:00.000Z' WHERE week_start < '2027-01-04'; UPDATE shopping_groups SET frozen_dinner_name = (SELECT name FROM dinners WHERE dinners.id = dinner_id) WHERE list_id IN (SELECT id FROM shopping_lists WHERE frozen_at IS NOT NULL)`); err != nil {
 		t.Fatal(err)
 	}