feat: notifications system, rate limiting, fix recipe visibility 404, follow race

Part of the social-feature backlog (follow, comments, reactions, ratings,
feed, threading) audited earlier — see conversation.

- notifications table: follow/comment/reply/reaction/rating events,
  replaces the fully-dead feed_items table (feed_item_type enum existed
  but had zero references anywhere in the codebase).
- Bell UI in the nav with unread badge, mark-all-read, 30s poll.
- Rate limiting on comment posting (20/min), follow/unfollow (30/min),
  and comment reactions (60/min) — previously unthrottled.
- /recipes/[id] queried by (id, authorId=session.user) only, so any
  recipe not owned by the viewer 404'd regardless of visibility.
  Widen the query to include public/unlisted recipes and gate the
  owner-only actions (edit, delete, version history, translate, AI
  content generation) behind an isOwner check.
- user_follows had no primary key/unique constraint, so the follow
  route's onConflictDoNothing() was a silent no-op — concurrent follow
  clicks could insert duplicate rows and inflate follower counts. Add
  a composite primary key on (follower_id, following_id).

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

apps/web/messages/fr.json

@@ -14,7 +14,18 @@
     "signOut": "Se déconnecter",
     "apiKeys": "Clés API",
     "webhooks": "Webhooks",
-    "language": "Langue"
+    "language": "Langue",
+    "notifications": "Notifications"
+  },
+  "notifications": {
+    "title": "Notifications",
+    "empty": "Aucune notification pour l'instant.",
+    "markAllRead": "Tout marquer comme lu",
+    "follow": "{name} vous suit désormais",
+    "comment": "{name} a commenté votre recette",
+    "reply": "{name} a répondu à votre commentaire",
+    "reaction": "{name} a réagi à votre commentaire",
+    "rating": "{name} a noté votre recette"
   },
   "recipe": {
     "share": "Partager",