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>

packages/db/src/migrations/meta/_journal.json

@@ -113,6 +113,27 @@
       "when": 1783105214683,
       "tag": "0015_aromatic_lester",
       "breakpoints": true
+    },
+    {
+      "idx": 16,
+      "version": "7",
+      "when": 1783107462796,
+      "tag": "0016_broken_nemesis",
+      "breakpoints": true
+    },
+    {
+      "idx": 17,
+      "version": "7",
+      "when": 1783108057675,
+      "tag": "0017_shallow_malcolm_colcord",
+      "breakpoints": true
+    },
+    {
+      "idx": 18,
+      "version": "7",
+      "when": 1783108229117,
+      "tag": "0018_mighty_butterfly",
+      "breakpoints": true
     }
   ]
 }