ci: sync VERSION file back to default branch after release

.github/workflows/release.yml

@@ -271,3 +271,36 @@ jobs:
                 parse_mode: "Markdown",
                 disable_web_page_preview: true
               }')"
+
+  sync-version-file:
+    needs: [release]
+    if: ${{ needs.release.result == 'success' }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout default branch
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ github.event.repository.default_branch }}
+
+      - name: Sync VERSION file to released tag
+        run: |
+          if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
+            VERSION=${{ github.event.inputs.tag }}
+            VERSION=${VERSION#v}
+          else
+            VERSION=${GITHUB_REF#refs/tags/v}
+          fi
+
+          CURRENT_VERSION=$(tr -d '\r\n' < backend/cmd/server/VERSION || true)
+          if [ "$CURRENT_VERSION" = "$VERSION" ]; then
+            echo "VERSION file already matches $VERSION"
+            exit 0
+          fi
+
+          echo "$VERSION" > backend/cmd/server/VERSION
+
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add backend/cmd/server/VERSION
+          git commit -m "chore: sync VERSION to ${VERSION} [skip ci]"
+          git push origin HEAD:${{ github.event.repository.default_branch }}