feat(scrobble stats): genre ranking (Top-Genres) — web + android

Play-log now stores each play's genre (from the song tags; web scrobbleLog +
android PlayLogClient capture song.genre). The stats endpoint returns topGenres
(GROUP BY lower(genre), case-insensitive, blanks ignored); web + android render a
Top-Genres section. Schema migrates via ALTER TABLE. Old plays have no genre
(fills going forward, like cover art). SW cache -> ns-v72.

Android compileDebugKotlin + web build green; genre grouping + null-ignore unit-tested.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scrobble/scrobble-server.js

@@ -53,19 +53,21 @@ db.exec(`
     played_at  INTEGER NOT NULL,   -- Unix epoch, UTC
     source     TEXT,               -- 'web' | 'android'
     uuid       TEXT UNIQUE,        -- idempotency for offline-retry
-    cover_art  TEXT                -- Subsonic coverArt token (NOT the entity id) for real covers
+    cover_art  TEXT,               -- Subsonic coverArt token (NOT the entity id) for real covers
+    genre      TEXT                -- per-play genre (from the song's tags) for genre stats
   );
 `);
-// Migration for DBs created before cover_art existed (ALTER throws if the column is already there).
+// Migrations for DBs created before a column existed (ALTER throws if it's already there).
 try { db.exec('ALTER TABLE plays ADD COLUMN cover_art TEXT'); } catch {}
+try { db.exec('ALTER TABLE plays ADD COLUMN genre TEXT'); } catch {}
 db.exec('CREATE INDEX IF NOT EXISTS ix_plays_user_time   ON plays(user, played_at);');
 db.exec('CREATE INDEX IF NOT EXISTS ix_plays_user_artist ON plays(user, artist_id);');
 db.exec('CREATE INDEX IF NOT EXISTS ix_plays_user_song   ON plays(user, song_id);');
 
 const _insert = db.prepare(`
   INSERT OR IGNORE INTO plays
-    (user, song_id, title, artist, artist_id, album, album_id, duration, played_at, source, uuid, cover_art)
-  VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+    (user, song_id, title, artist, artist_id, album, album_id, duration, played_at, source, uuid, cover_art, genre)
+  VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
 `);
 const _history = db.prepare(`
   SELECT id, song_id, title, artist, artist_id, album, album_id, duration, played_at, source
@@ -145,7 +147,8 @@ app.post('/scrobble/plays', async (req, res) => {
         _toEpochSec(p.playedAt),
         p.source === 'android' ? 'android' : 'web',
         uuid,
-        p.coverArt ?? null
+        p.coverArt ?? null,
+        p.genre ?? null
       );
       if (r.changes > 0) accepted++; else duplicates++;
     } catch { invalid++; }
@@ -220,6 +223,12 @@ app.get('/scrobble/stats', async (req, res) => {
     GROUP BY song_id ORDER BY plays DESC LIMIT :limit
   `).all(winL);
 
+  const topGenres = db.prepare(`
+    SELECT MAX(genre) AS name, COUNT(*) AS plays
+    FROM plays WHERE ${WHERE} AND genre IS NOT NULL AND genre <> ''
+    GROUP BY lower(genre) ORDER BY plays DESC LIMIT :limit
+  `).all(winL);
+
   const series = db.prepare(`
     SELECT strftime('${fmt}', played_at + :tz, 'unixepoch') AS bucket, COUNT(*) AS plays, COALESCE(SUM(duration),0) AS seconds
     FROM plays WHERE ${WHERE} GROUP BY bucket ORDER BY bucket
@@ -238,7 +247,7 @@ app.get('/scrobble/stats', async (req, res) => {
     firstPlayedAt,
     totalPlays: totals.plays,
     totalSeconds: totals.seconds,
-    topArtists, topAlbums, topTracks, series, byHour,
+    topArtists, topAlbums, topTracks, topGenres, series, byHour,
   });
 });