feat(recommendations): content-based "Recommended for you" endpoint

Add GET /me/recommendations: rank mangas by weighted tag overlap with the
user's taste — explicit like +1.0, bookmark +0.5, dislike −1.0 (reaction
overrides bookmark). Per-tag affinities are summed, candidates scored by
their tags' affinity normalized by tag count (anti-tag-stuffing, à la
list_similar), with already-reacted/bookmarked/read mangas excluded and
net-negative candidates dropped (dislike down-ranks, not browse-hides).
Reuses manga_cols/cards_from_rows. Integration tests cover like-driven recs,
dislike down-rank, bookmark half-weight ordering, seen-exclusion, empty
cold-start, and auth.

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

backend/src/api/mangas.rs

@@ -22,6 +22,7 @@ pub fn routes() -> Router<AppState> {
         .route("/mangas", get(list).post(create))
         .route("/mangas/:id", get(get_one).patch(update))
         .route("/mangas/:id/similar", get(list_similar))
+        .route("/me/recommendations", get(list_recommendations))
         .route("/mangas/:id/cover", put(put_cover).delete(delete_cover))
         .route("/mangas/:id/tags", post(attach_tag))
         .route("/mangas/:id/tags/:tag_id", delete(detach_tag))
@@ -188,6 +189,28 @@ async fn list_similar(
     Ok(Json(json!({ "items": items })))
 }
 
+const RECOMMENDATIONS_LIMIT: i64 = 12;
+
+#[derive(Debug, Deserialize)]
+pub struct RecommendationParams {
+    #[serde(default)]
+    pub limit: Option<i64>,
+}
+
+/// `GET /api/v1/me/recommendations` — content-based "Recommended for you",
+/// ranked by tag overlap with the signed-in user's likes/bookmarks (minus
+/// dislikes). Returns `{ "items": [...] }` (a fixed top-N, like `/similar`);
+/// empty when the user has no taste signals yet.
+async fn list_recommendations(
+    State(state): State<AppState>,
+    CurrentUser(user): CurrentUser,
+    Query(params): Query<RecommendationParams>,
+) -> AppResult<Json<serde_json::Value>> {
+    let limit = params.limit.unwrap_or(RECOMMENDATIONS_LIMIT).clamp(1, 50);
+    let items = repo::manga::list_recommendations(&state.db, user.id, limit).await?;
+    Ok(Json(json!({ "items": items })))
+}
+
 /// `POST /api/v1/mangas` is multipart/form-data. Parts:
 ///
 /// - `metadata` (required): JSON body matching `NewManga` — title, optional