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>

backend/migrations/0035_manga_reactions.sql

@@ -0,0 +1,15 @@
+-- Per-user like/dislike reactions on mangas — a private taste signal that
+-- powers content-based recommendations. One row per (user, manga); the
+-- `reaction` column toggles between 'like' and 'dislike', and clearing a
+-- reaction deletes the row. Reactions are never exposed publicly (no counts).
+CREATE TABLE manga_reactions (
+    user_id    uuid NOT NULL REFERENCES users(id)  ON DELETE CASCADE,
+    manga_id   uuid NOT NULL REFERENCES mangas(id) ON DELETE CASCADE,
+    reaction   text NOT NULL CHECK (reaction IN ('like', 'dislike')),
+    created_at timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, manga_id)
+);
+
+-- Recommendations aggregate a user's reacted mangas by tag; the PK covers
+-- per-user lookups, this covers the reverse (all reactions on a manga).
+CREATE INDEX manga_reactions_manga_idx ON manga_reactions (manga_id);