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/fr.json

@@ -41,7 +41,9 @@
     "detail": {
       "comment": "{name} a commenté votre recette « {title} »",
       "rating": "{name} a noté votre recette « {title} » {stars} étoiles",
-      "leftoverExpiring": "Votre « {dish} » de « {title} » arrive à expiration — à manger vite !"
+      "leftoverExpiring": "Votre « {dish} » de « {title} » arrive à expiration — à manger vite !",
+      "shoppingListChecked": "{name} a coché « {item} » dans « {list} »",
+      "shoppingListItemsAdded": "{name} a ajouté {countLabel} à « {list} »"
     },
     "pushTitle": {
       "follow": "Nouvel abonné",
@@ -50,7 +52,8 @@
       "reaction": "Nouvelle réaction",
       "rating": "Nouvelle note",
       "mention": "Vous avez été mentionné",
-      "leftoverExpiring": "Restes bientôt périmés"
+      "leftoverExpiring": "Restes bientôt périmés",
+      "shoppingListUpdate": "Mise à jour de la liste de courses"
     }
   },
   "recipe": {