feat: add callsign to DLD award entity details

The DLD award detail page was only showing the mode in QSO entries
because the entity breakdown didn't include the callsign field.

Changes:
- Backend: Add callsign field to DOK award entity details

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/backend/services/awards.service.js

@@ -216,6 +216,7 @@ async function calculateDOKAwardProgress(userId, award, options = {}) {
         entityName: dok,
         band,
         mode,
+        callsign: qso.callsign,
         worked: false,
         confirmed: false,
         qsoDate: qso.qsoDate,