Generate the week's Supply run from its plan

Add internal/shopping with Generate(weekStart), one entry point for the
first generation and every regeneration. It builds one group per plan
day (dinner_id, date, position by date) and one item per ingredient of
the dinner's recipe, in ingredient order, reusing the values recipe
grounding stored: a fetched recipe's normalized name with qty and unit
joined for display, or a manual recipe's line as typed with no qty. A
day whose dinner has no recipe or no ingredients gets no group and is
reported in Skipped. Regenerating keeps the id and needed flag of each
item whose (dinner_id, name) is unchanged, matching repeated names in
order; new items are needed and items no longer planned are removed. It
is deterministic and makes no model call. A frozen list is refused.

Migration 0016 makes shopping_lists.title nullable (never empty) so a
list exists untitled until the AI titles it, and adds shopping_groups.date.

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

internal/db/models.go

@@ -258,6 +258,7 @@ type ShoppingGroup struct {
 	ID        string
 	ListID    string
 	DinnerID  string
+	Date      string
 	Position  int64
 	CreatedAt string
 	UpdatedAt string
@@ -278,7 +279,7 @@ type ShoppingList struct {
 	ID           string
 	WeekStart    string
 	PlanID       string
-	Title        string
+	Title        sql.NullString
 	Status       string
 	SentToChatAt sql.NullString
 	FrozenAt     sql.NullString