ui: cover art in Beatport, Spotify e YouTube search

- core/beatport.py: BeatportTrack.image_url estratto da image.dynamic_uri (95x95)
- core/spotify_client.py: _track_to_dict include image_url (Spotify torna 3 taglie,
  prendo la ~64px). search_artist_discography inietta album.images anche negli
  album-tracks per la cover
- core/youtube_search.py: image_url = i.ytimg.com/vi/<id>/mqdefault.jpg
- webui/index.html: nuova <th class="col-cover"></th> nelle 3 tabelle
- webui/js/app.js: 3 renderTable inseriscono <td class="col-cover"><img>
  con loading=lazy, referrerpolicy=no-referrer e onerror fallback
- webui/css/style.css: .beatport-table .col-cover img { 40x40, border-radius 4px,
  object-fit: cover, bg fallback var(--bg-input) }

core/youtube_search.py

@@ -62,11 +62,13 @@ def search_youtube(query: str, limit: int = 50) -> list:
             continue
         video_id = e.get("id") or ""
         url = e.get("url") or (f"https://www.youtube.com/watch?v={video_id}" if video_id else "")
+        image_url = f"https://i.ytimg.com/vi/{video_id}/mqdefault.jpg" if video_id else ""
         result.append({
             "id": video_id,
             "url": url,
             "title": e.get("title") or "",
             "channel": e.get("uploader") or e.get("channel") or "",
             "duration_sec": int(e.get("duration") or 0),
+            "image_url": image_url,
         })
     return result