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/ci.yml

@@ -0,0 +1,64 @@
+name: PR checks
+
+# Runs on pull requests into main (typically from dev) and into dev (typically
+# from feature/* branches): lint + format checks and a no-push image build to
+# prove the Docker images still build.
+
+on:
+  pull_request:
+    branches: [main, dev]
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    name: Lint & format
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install ruff
+        run: pip install "ruff>=0.8.0"
+
+      - name: Ruff lint (Python)
+        run: ruff check .
+
+      - name: Ruff format check (Python)
+        run: ruff format --check .
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install frontend deps
+        run: npm ci
+        working-directory: frontend
+
+      - name: ESLint (frontend)
+        run: npm run lint
+        working-directory: frontend
+
+      - name: Prettier check (frontend)
+        run: npm run format:check
+        working-directory: frontend
+
+  build:
+    name: Docker build
+    uses: ./.github/workflows/_docker-build-push.yml
+    with:
+      push: false
+      tag_mode: dev