feat(scrobble): time-travel navigator (year/month) + fix stats cover art

Navigator (web + android): browse Alles / Jahr / Monat with a ‹ › stepper — see
top plays for a specific month or year (e.g. March 2024). Steps into the future
are disabled. Uses the existing /scrobble/stats endpoint with computed from/to +
bucket (month for year view, day for month view). All windows verified.

Cover art fix: the app resolves covers from the Subsonic **coverArt token**, not
the raw entity id — so stats covers (which used albumId/songId) never loaded.
Now the play-log stores each play's coverArt (web + android capture), the stats
endpoint returns it for top tracks/albums, and both clients render coverUrl(coverArt)
with an id fallback. Old plays lack coverArt (blank until re-listened); artist
images still use artistId (may need getArtistInfo2 later if blank). Schema migrates
existing DBs via ALTER TABLE. SW cache -> ns-v69.

Android compileDebugKotlin + web build green; server windows + cover storage unit-tested.

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

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v68';
+const CACHE = 'ns-v69';
 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.