feat(ci): trigger Watchtower update after image push

Automatically notify Watchtower to pull and deploy the new image
after a successful registry push.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/build.yml

@@ -27,3 +27,9 @@ jobs:
           tags: |
             git.schweitz.internal/jpmschweitzer/tatlock:latest
             git.schweitz.internal/jpmschweitzer/tatlock:${{ github.ref_name }}
+
+      - name: Trigger Watchtower update
+        if: success()
+        run: |
+          curl -sf -H "Authorization: Bearer ${{ secrets.WATCHTOWER_TOKEN }}" \
+            http://watchtower:8080/v1/update