feat(reco): genre backfill + reco-backed Rediscover row

Wire the reco store into the For You favorite-albums branch:
- Genre backfill: when favorite albums resolve (full Album with genre), write
  each (genre_id, genre_name) onto its reco events + album-meta via
  spawn_blocking. Plays carry no genre, so this makes the engine's get_top_genres
  non-empty (Tauri never backfilled, so its top-genres was dead).
- Rediscover: build_rediscover prefers the reco 'forgotten favorites' set
  (favorited, not played in 30d, from the Tauri-shared events.db) when warm,
  falling back to the local recents heuristic when cold so the row never empties
  (non-regressing for fresh stores, better for Tauri-migrants).

cc

crates/qbz/src/foryou.rs

@@ -420,11 +420,21 @@ fn top_artist_slims(fav_artists: &[Artist]) -> Vec<ArtistSlim> {
         .collect()
 }
 
-/// Rediscover — favorite albums not in the recent play-history.
-fn build_rediscover(fav_albums: &[Album], recent_ids: &HashSet<String>) -> Vec<AlbumCard> {
+/// Rediscover — favorite albums the user hasn't returned to lately. Prefers the
+/// reco store's "forgotten favorites" (favorited, not played in 30d, from the
+/// Tauri-shared events.db) when warm; falls back to "not in the local recents
+/// cache" when reco is cold so the row never empties.
+fn build_rediscover(
+    fav_albums: &[Album],
+    recent_ids: &HashSet<String>,
+    forgotten: Option<&HashSet<String>>,
+) -> Vec<AlbumCard> {
     fav_albums
         .iter()
-        .filter(|a| !recent_ids.contains(&a.id))
+        .filter(|a| match forgotten {
+            Some(set) => set.contains(&a.id),
+            None => !recent_ids.contains(&a.id),
+        })
         .take(18)
         .cloned()
         .map(map_album)
@@ -854,7 +864,34 @@ pub fn spawn_for_you<A>(
             Box::pin(async move {
                 let fav_albums = fetch_fav_albums(&runtime).await;
                 apply_favorite_albums(&weak, &cache, build_favorite_albums(&fav_albums));
-                apply_rediscover(&weak, &cache, build_rediscover(&fav_albums, &recent_ids));
+                // reco: backfill genres for the resolved favorite albums so the
+                // engine's top-genres has data (plays alone carry no genre).
+                let genre_entries: Vec<(String, u64, String)> = fav_albums
+                    .iter()
+                    .filter_map(|a| {
+                        a.genre
+                            .as_ref()
+                            .filter(|g| g.id > 0)
+                            .map(|g| (a.id.clone(), g.id, g.name.clone()))
+                    })
+                    .collect();
+                if !genre_entries.is_empty() {
+                    tokio::task::spawn_blocking(move || {
+                        crate::reco::backfill_album_genres(genre_entries)
+                    });
+                }
+                // reco: prefer the reco "forgotten favorites" set when the store
+                // is warm (shared events.db); fall back to the local recents
+                // heuristic when cold so the Rediscover row never empties.
+                let forgotten: Option<HashSet<String>> =
+                    crate::reco::forgotten_favorite_album_ids(60, 30)
+                        .filter(|ids| !ids.is_empty())
+                        .map(|ids| ids.into_iter().collect());
+                apply_rediscover(
+                    &weak,
+                    &cache,
+                    build_rediscover(&fav_albums, &recent_ids, forgotten.as_ref()),
+                );
                 apply_radio(&weak, &cache, build_radio(&recent_album_list, &fav_albums));
 
                 // Only the no-recent-history case needs the favorite-album seed;

crates/qbz/src/reco.rs

@@ -179,6 +179,39 @@ pub fn log_playlist_add(playlist_id: Option<u64>, track_ids: Vec<u64>) {
     }
 }
 
+// ---------------------------------------------------------------------------
+// Surfaces (W7/W8)
+// ---------------------------------------------------------------------------
+
+/// Forgotten-favorite album ids (favorited, not played within `recency_days`).
+/// `None` when reco is disabled or the read fails — the caller falls back to
+/// its local heuristic so the Rediscover row never empties on a cold store.
+pub fn forgotten_favorite_album_ids(limit: u32, recency_days: u32) -> Option<Vec<String>> {
+    let guard = RECO.lock().ok()?;
+    let store = guard.as_ref()?;
+    store
+        .get_forgotten_favorite_album_ids(limit, recency_days)
+        .ok()
+}
+
+/// Backfill genres `(album_id, genre_id, genre_name)` onto reco events +
+/// album-meta once albums are resolved. Best-effort, blocking SQLite — call
+/// from `spawn_blocking`. Plays carry no genre, so this is what feeds
+/// `get_top_genres`; idempotent (only fills still-NULL event genres).
+pub fn backfill_album_genres(entries: Vec<(String, u64, String)>) {
+    if entries.is_empty() {
+        return;
+    }
+    if let Ok(guard) = RECO.lock() {
+        if let Some(store) = guard.as_ref() {
+            for (album_id, genre_id, genre_name) in entries {
+                let _ = store.update_genre_for_album(&album_id, genre_id);
+                let _ = store.set_album_genre_name(&album_id, &genre_name);
+            }
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Training (W5)
 // ---------------------------------------------------------------------------