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/src/api/mod.rs

@@ -11,6 +11,7 @@ pub mod history;
 pub mod mangas;
 pub mod page_tags;
 pub mod pagination;
+pub mod reactions;
 pub mod tags;
 
 use axum::Router;
@@ -31,5 +32,6 @@ pub fn routes() -> Router<AppState> {
         .merge(collections::routes())
         .merge(page_tags::routes())
         .merge(history::routes())
+        .merge(reactions::routes())
         .merge(admin::routes())
 }