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/store/schema_test.go

@@ -136,6 +136,7 @@ func testRow(table string, i int) map[string]any {
 	case "shopping_groups":
 		r["list_id"] = rowID("shopping_lists", 0)
 		r["dinner_id"] = rowID("dinners", 0)
+		r["date"] = fmt.Sprintf("2026-01-%02d", i+5)
 		r["position"] = i
 	case "shopping_items":
 		r["group_id"] = rowID("shopping_groups", 0)