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>

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v71';
+const CACHE = 'ns-v72';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.