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/components/layout/nav.tsx

@@ -21,6 +21,7 @@ import {
   SheetTrigger,
 } from "@/components/ui/sheet";
 import { Avatar, AvatarFallback, AvatarImage } from "@/components/ui/avatar";
+import { NotificationBell } from "@/components/social/notification-bell";
 import { authClient } from "@/lib/auth/client";
 import { useTranslations } from "next-intl";
 
@@ -100,6 +101,7 @@ export function Nav() {
           ))}
         </nav>
         <div className="ml-auto flex items-center gap-2">
+          <NotificationBell />
           <DropdownMenu>
             <DropdownMenuTrigger className="rounded-full outline-none focus-visible:ring-2 focus-visible:ring-ring">
               <Avatar className="h-8 w-8">