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/security.yml

@@ -0,0 +1,89 @@
+name: Security
+
+# Audits the full dependency set (pip-audit, npm audit) and runs a Trivy
+# filesystem scan (dependencies + Dockerfile/IaC misconfig). Trivy results are
+# uploaded to the Security -> Code scanning tab.
+
+on:
+  pull_request:
+    branches: [main, dev]
+  push:
+    branches: [main, dev]
+  schedule:
+    - cron: "27 4 * * 1" # weekly, Monday 04:27 UTC
+  workflow_dispatch:
+
+concurrency:
+  group: security-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  python-audit:
+    name: pip-audit
+    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 pip-audit
+        run: pip install pip-audit
+
+      - name: Audit Python dependencies
+        run: pip-audit -r backend/requirements.txt
+
+  npm-audit:
+    name: npm audit
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - 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: Audit npm dependencies
+        run: npm audit --audit-level=high
+        working-directory: frontend
+
+  trivy:
+    name: Trivy filesystem scan
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      security-events: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run Trivy
+        uses: aquasecurity/trivy-action@0.28.0
+        with:
+          scan-type: fs
+          scan-ref: .
+          format: sarif
+          output: trivy-results.sarif
+          severity: CRITICAL,HIGH
+          ignore-unfixed: true
+
+      - name: Upload Trivy results
+        uses: github/codeql-action/upload-sarif@v3
+        if: always()
+        with:
+          sarif_file: trivy-results.sarif
+          category: trivy-fs