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/messages/en.json

@@ -368,7 +368,9 @@
   },
   "publicShoppingList": {
     "sharedList": "Shared shopping list",
-    "wantYourOwn": "Want your own shopping lists?"
+    "wantYourOwn": "Want your own shopping lists?",
+    "anonymousEditor": "Someone with the link",
+    "editableBadge": "Anyone with this link can edit"
   },
   "publicRecipe": {
     "publicRecipeBy": "Public recipe by",
@@ -1018,6 +1020,8 @@
     "publicLinkTitle": "Public link",
     "publicLinkDescription": "Anyone with the link can view this list.",
     "publicLinkToggleFailed": "Could not update the public link",
+    "publicEditableTitle": "Allow editing",
+    "publicEditableDescription": "Anyone with the link can check off, add, and reorder items — no account needed.",
     "copyLink": "Copy link",
     "linkCopied": "Link copied",
     "copyLinkFailed": "Could not copy link"