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/db/models.go

@@ -255,13 +255,14 @@ type Session struct {
 }
 
 type ShoppingGroup struct {
-	ID        string
-	ListID    string
-	DinnerID  string
-	Date      string
-	Position  int64
-	CreatedAt string
-	UpdatedAt string
+	ID               string
+	ListID           string
+	DinnerID         string
+	Date             string
+	Position         int64
+	CreatedAt        string
+	UpdatedAt        string
+	FrozenDinnerName sql.NullString
 }
 
 type ShoppingItem struct {