feat(09-04): add Slack notification on pipeline failure

- Add notify job that runs when test or build fails
- Use curl to Slack webhook for Gitea compatibility
- Notify job depends on both test and build jobs

.gitea/workflows/build.yaml

@@ -102,3 +102,16 @@ jobs:
           git add helm/taskplaner/values.yaml
           git commit -m "chore: update image tag to ${SHORT_SHA} [skip ci]" || echo "No changes to commit"
           git push || echo "Push failed - may need to configure git credentials"
+
+  notify:
+    needs: [test, build]
+    runs-on: ubuntu-latest
+    if: failure()
+    steps:
+      - name: Notify Slack on failure
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+        run: |
+          curl -X POST -H 'Content-type: application/json' \
+            --data "{\"text\":\"Pipeline failed for ${{ gitea.repository }} on ${{ gitea.ref }}\"}" \
+            $SLACK_WEBHOOK_URL