ci: bump checkout to v6, upload-artifact to v7 (Node.js 24 ready)

GitHub-Warnung: v4 dieser Actions laeuft auf Node.js 20, das wird
im Sept 2026 abgeschaltet. checkout@v6 und upload-artifact@v7
sind die aktuellen Node.js-24-Versionen. Funktional identisch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/nostr-sync.yml

@@ -19,13 +19,13 @@ jobs:
 
     steps:
       - name: Checkout content repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
           path: content-repo
 
       - name: Checkout mdparser
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: edufeed-org/mdparser
           ref: main
@@ -77,7 +77,7 @@ jobs:
 
       - name: Upload run log
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: nostr-sync-log-${{ github.run_id }}
           path: mdparser/sync/logs/publish-*.json