feat: replace Bandsintown with Ticketmaster for concert events

Backend: swap /api/events/:artist to use Ticketmaster Discovery API
(startDateTime=now, classificationName=music, size=20, sort by date).
Maps TM response to the same {datetime, venue, offers} shape the modal
expects. Returns [] if TICKETMASTER_API_KEY is not set.

Frontend: remove all debug console.logs from events.js; drop redundant
client-side date filter (TM already filters by startDateTime server-side).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/lib/config.js

@@ -9,7 +9,8 @@ const CONFIG = {
   // 'ytdlp' | 'squid' | 'auto' (squid first, yt-dlp fallback)
   downloadBackend: (process.env.DOWNLOAD_BACKEND || 'auto').toLowerCase(),
   // squid.wtf quality: 'mp3' | 'flac' | 'hi-res'
-  squidQuality:    (process.env.SQUID_QUALITY   || 'mp3').toLowerCase(),
+  squidQuality:       (process.env.SQUID_QUALITY        || 'mp3').toLowerCase(),
+  ticketmasterKey:     process.env.TICKETMASTER_API_KEY  || '',
 };
 
 module.exports = CONFIG;