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/lib/notifications.ts

@@ -0,0 +1,22 @@
+import { db, notifications } from "@epicure/db";
+import { randomUUID } from "crypto";
+
+type NotificationType = "follow" | "comment" | "reply" | "reaction" | "rating" | "mention";
+
+export async function createNotification(opts: {
+  userId: string;
+  type: NotificationType;
+  actorId: string;
+  recipeId?: string;
+  commentId?: string;
+}): Promise<void> {
+  if (opts.userId === opts.actorId) return; // never notify yourself
+  await db.insert(notifications).values({
+    id: randomUUID(),
+    userId: opts.userId,
+    type: opts.type,
+    actorId: opts.actorId,
+    recipeId: opts.recipeId,
+    commentId: opts.commentId,
+  });
+}