feat: public shopping list links can allow editing

Owner opts in per-list via a new "Allow editing" toggle next to the
existing public-link switch. Anonymous writes are scoped to that one
list only — the link id is the sole credential, enforced in
getShoppingListAccess and the item routes (no session required there
now), with an IP rate limit on genuinely anonymous requests. Turning
off the public link also revokes editing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/api-auth.ts

@@ -13,6 +13,12 @@ export async function requireSession() {
   return { session, response: null };
 }
 
+/** Like requireSession, but never 401s — for endpoints that also accept anonymous
+ *  access via a resource-scoped capability (e.g. a public-editable share link). */
+export async function getOptionalSession() {
+  return auth.api.getSession({ headers: await headers() });
+}
+
 export async function requireAdmin() {
   const { session, response } = await requireSession();
   if (response) return { session: null, response };