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/notify_test.go

@@ -152,7 +152,7 @@ func TestSendToChatFailuresRecordNothing(t *testing.T) {
 		t.Errorf("after failed sends sent_to_chat_at = %v, chat_send_unconfirmed_at = %v, %v, want both unset", l.SentToChatAt, l.ChatSendUnconfirmedAt, err)
 	}
 
-	if _, err := e.st.Writer.Exec(`UPDATE shopping_lists SET status = 'archived', frozen_at = '2026-10-19T04:05:00Z'`); err != nil {
+	if _, err := e.st.Writer.Exec(`UPDATE shopping_lists SET status = 'archived', frozen_at = '2026-10-19T04:05:00Z'; 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)
 	}
 	posts := len(e.chat.Posts())