fix: reliable concerts (backend events cache) + web importer preview layout

- Backend now caches Ticketmaster events per artist and prefers the larger
  recent result, so a flaky partial/empty response can't shrink a good list;
  every client (web/Android) stays consistent (1h TTL self-heals).
- Web SoundCloud importer preview: add the missing .gtrack-num grid cell so
  the 4-column row no longer squeezes the cover/title (title was wrapping one
  char per line).
- CHANGELOG [2.3] opened for this batch.

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

src/views/soundcloud.js

@@ -138,6 +138,7 @@ function _renderSoundCloudPreview(data) {
     const artT = t.artworkUrl || '';
     el.innerHTML = `
       <div class="gtrack-art">${artT ? `<img src="${artT}" loading="lazy">` : '<div style="width:100%;height:100%;display:flex;align-items:center;justify-content:center;font-size:22px">🎵</div>'}</div>
+      <div class="gtrack-num">${i + 1}</div>
       <div class="gtrack-info">
         <div class="gtrack-title">${esc(t.trackName || '—')}</div>
         <div class="gtrack-sub">${esc(t.artistName || '—')}${t.genre ? ' · ' + esc(t.genre) : ''}${t.duration ? ' · ' + Math.round(t.duration / 60) + ' min' : ''}</div>