Add merge-backport workflow to automate PR backports after successful builds

Signed-off-by: Chaoscaot <max@maxsp.de>

.gitea/workflows/pull-request-build.yml

@@ -39,3 +39,33 @@ jobs:
           echo "$SW_MAVEN_CREDENTIALS" > steamwar.properties
       - name: Build with Gradle
         run: ./gradlew build --no-daemon
+
+  merge-backport:
+    name: Merge backport
+    runs-on: ubuntu-latest
+    needs: build
+    if: ${{ startsWith(github.event.pull_request.base.ref, 'version/') && startsWith(github.event.pull_request.head.ref, 'backport/pr-') }}
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - name: Merge successful backport PR
+        shell: bash
+        env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+        run: |
+          set -euo pipefail
+
+          pr_number="$(jq -r '.number' "$GITHUB_EVENT_PATH")"
+          target_branch="$(jq -r '.pull_request.base.ref' "$GITHUB_EVENT_PATH")"
+          payload="$(jq -n \
+            --arg title "Merge backport #${pr_number} into ${target_branch}" \
+            '{Do: "merge", MergeTitleField: $title, MergeMessageField: "Automatic CommonCore backport after successful build.", delete_branch_after_merge: true}')"
+
+          curl --fail --silent --show-error \
+            -X POST \
+            -H "Authorization: token ${GITEA_TOKEN}" \
+            -H "Accept: application/json" \
+            -H "Content-Type: application/json" \
+            --data "$payload" \
+            "${GITHUB_API_URL}/repos/${GITHUB_REPOSITORY}/pulls/${pr_number}/merge"