fix(ci): docs-sync, electron linux fpm, docker cache env

CI Lint fixes:
- docs/openapi.yaml: bump version 2.2.0 → 2.2.3 (was out of sync with package.json)
- CHANGELOG.md: add '## [Unreleased]' as first section (required by check:docs-sync)

Electron Linux fix:
- electron-release.yml: add 'gem install fpm' step for Linux builds
  fpm is required by electron-builder to package .deb installers;
  ubuntu-latest runners don't have it pre-installed

Docker publish:
- docker-publish.yml: add DOCKER_BUILDKIT_INLINE_CACHE env; prev 502 was
  a transient Docker Hub network error, no code change needed

.github/workflows/docker-publish.yml

@@ -49,6 +49,9 @@ jobs:
             ${{ env.IMAGE_NAME }}:latest
           cache-from: type=gha
           cache-to: type=gha,mode=max
+          no-cache: false
+        env:
+          DOCKER_BUILDKIT_INLINE_CACHE: 1
 
       - name: Inspect image
         run: |

.github/workflows/electron-release.yml

@@ -107,6 +107,10 @@ jobs:
           "
           echo "✓ electron/package.json version set to $VERSION_NO_V"
 
+      - name: Install fpm (Linux .deb packaging tool)
+        if: matrix.platform == 'linux'
+        run: sudo gem install fpm --no-document
+
       - name: Install Electron dependencies
         working-directory: electron
         run: npm install --no-audit --no-fund