ci: pushover notification on pipeline failure

Uses pushover_token and pushover_user secrets configured in Woodpecker.
Step runs only on failure (when.status: failure) so successful runs stay
quiet. Message includes branch, commit message, and a link back to the
pipeline.

.woodpecker.yml

@@ -16,3 +16,23 @@ steps:
     image: node:22-alpine
     commands:
       - npm run build
+
+  - name: notify-failure
+    image: curlimages/curl:latest
+    environment:
+      PUSHOVER_TOKEN:
+        from_secret: pushover_token
+      PUSHOVER_USER:
+        from_secret: pushover_user
+    commands:
+      - |
+        curl -s \
+          --form-string "token=$PUSHOVER_TOKEN" \
+          --form-string "user=$PUSHOVER_USER" \
+          --form-string "title=Halfstreet CI failed" \
+          --form-string "message=$CI_COMMIT_BRANCH: $CI_COMMIT_MESSAGE" \
+          --form-string "url=$CI_PIPELINE_URL" \
+          --form-string "url_title=View pipeline" \
+          https://api.pushover.net/1/messages.json
+    when:
+      - status: failure