Add GetSongBPM backlink

public/index.html

@@ -69,6 +69,10 @@
 
     <div class="section-title">Up Next</div>
     <div class="suggestions" id="suggestions"></div>
+    
+    <div style="text-align: center; margin-top: 40px; font-family: 'JetBrains Mono', monospace; font-size: 10px; color: var(--text-dim);">
+      BPM & Key data provided by <a href="https://getsongbpm.com" target="_blank" style="color: var(--text-dim); text-decoration: underline;">GetSongBPM</a>
+    </div>
   </div>
 
   <script src="../shared/mixing-rules.js"></script>