Add Re-upload to Gokapi button for existing stems

- New reprocess_stems admin action triggers Gokapi upload manually
- Button appears in Stems/Extras Link section when stems_url exists
  but stems_link is empty (webhook fired before Gokapi was configured)
- Updated share link description and placeholder for Gokapi URLs

app.py

@@ -1145,6 +1145,18 @@ def admin_request(rid):
             flash('Stems share link saved.', 'success')
             return redirect(url_for('admin_request', rid=rid))
 
+        elif action == 'reprocess_stems':
+            # Manually trigger Gokapi upload for existing stems (e.g. webhook fired before Gokapi was configured).
+            if not req.get('stems_url'):
+                flash('No stems files to process.', 'error')
+                return redirect(url_for('admin_request', rid=rid))
+            gokapi_url, gokapi_err = process_stems_to_gokapi(req)
+            if gokapi_err:
+                flash(f'Failed to upload stems to Gokapi: {gokapi_err}', 'error')
+            else:
+                flash(f'Stems uploaded to Gokapi: {gokapi_url}', 'success')
+            return redirect(url_for('admin_request', rid=rid))
+
         elif action == 'cancel_request':
             update_request(rid, status='cancelled')
             flash('Request marked as cancelled.', 'success')