ci: add GitHub Actions for image builds, PR checks and security scans

- dev-images: build/push backend, worker, frontend to ghcr.io and Docker
  Hub on every push to dev (tags: dev, dev-<sha>)
- release-images: same images on v* tag push (tags: latest, <version>)
- PR checks (into main/dev): ruff lint+format, eslint+prettier, no-push
  Docker build of all targets
- CodeQL (python, js/ts), dependency-review, pip-audit, npm audit, Trivy fs
- pin .github/workflows/*.yml to LF

.github/workflows/docker-release.yml

@@ -0,0 +1,28 @@
+name: Release images
+
+# Build all three images and push them to ghcr.io and Docker Hub tagged
+# `latest` and the release version (the pushed tag with its leading `v`
+# stripped, e.g. v3.4.5 -> 3.4.5). Triggered only when a new v* tag is pushed,
+# so images are built once per release rather than on every commit to main.
+
+on:
+  push:
+    tags:
+      - "v*"
+  workflow_dispatch:
+
+concurrency:
+  group: docker-release-${{ github.ref }}
+  cancel-in-progress: false
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-push:
+    uses: ./.github/workflows/_docker-build-push.yml
+    with:
+      push: true
+      tag_mode: release
+    secrets: inherit