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/dependency-review.yml

@@ -0,0 +1,27 @@
+name: Dependency review
+
+# On PRs into main/dev, flag any newly added dependency that has a known
+# vulnerability or an incompatible license before it gets merged.
+
+on:
+  pull_request:
+    branches: [main, dev]
+
+permissions:
+  contents: read
+
+jobs:
+  dependency-review:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Dependency review
+        uses: actions/dependency-review-action@v4
+        with:
+          fail-on-severity: high
+          comment-summary-in-pr: on-failure