feat(reactions): per-user like/dislike on mangas

Add a private per-user reaction resource: a manga_reactions table (one row
per user+manga, like/dislike, cascades) and endpoints PUT/DELETE
/mangas/:id/reaction plus GET /me/reactions/:manga_id, wired on the existing
per-user seam (mirrors bookmarks/read_progress). Unknown manga → 404 via the
FK-violation mapping; an invalid reaction value → 422. Reactions are never
exposed publicly — this is a taste signal for the upcoming content-based
recommendations. Integration tests cover set/toggle/clear/read, per-user
isolation, and the 404/422/401 paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.105.0",
+  "version": "0.106.0",
   "private": true,
   "type": "module",
   "scripts": {