feat: push notification when a shared shopping list is updated

Shared shopping lists (owner + collaborator-invite members) had no
real-time coordination signal — no way to know someone else just checked
off milk or added items while you're mid-aisle. Notifies every other
member (excluding the actor) via push when an item is checked off or
items are added; no-op for solo unshared lists.

Deliberately skips the notifications table/notification-center (same
pattern as the existing leftover-expiry cron) — this is a lightweight
best-effort ping, not a persistent event, so it avoids a notification_type
enum migration and a new FK column for something transient.

Verified locally: added a real member to a shared list, registered a
push subscription, confirmed the check-off request's added latency
(945ms vs ~20ms baseline) shows the actual webpush call fired against
the recipient — individual subscription failures are swallowed by design
(Promise.allSettled in the existing sendPushNotification, unchanged);
confirmed a solo list stays fast (no recipients to notify, true no-op).

apps/web/messages/en.json

@@ -41,7 +41,9 @@
     "detail": {
       "comment": "{name} commented on your recipe \"{title}\"",
       "rating": "{name} rated your recipe \"{title}\" {stars} stars",
-      "leftoverExpiring": "Your \"{dish}\" from \"{title}\" is about to expire — eat it soon!"
+      "leftoverExpiring": "Your \"{dish}\" from \"{title}\" is about to expire — eat it soon!",
+      "shoppingListChecked": "{name} checked off \"{item}\" in \"{list}\"",
+      "shoppingListItemsAdded": "{name} added {countLabel} to \"{list}\""
     },
     "pushTitle": {
       "follow": "New follower",
@@ -50,7 +52,8 @@
       "reaction": "New reaction",
       "rating": "New rating",
       "mention": "You were mentioned",
-      "leftoverExpiring": "Leftovers expiring soon"
+      "leftoverExpiring": "Leftovers expiring soon",
+      "shoppingListUpdate": "Shopping list update"
     }
   },
   "recipe": {