Add security standards and CI infrastructure

- SECURITY.md with vulnerability disclosure policy and self-hosting guidance
- Dependabot config for weekly dependency updates (npm, Docker, Actions)
- ESLint v9 flat config for frontend and backend, with eslint-plugin-security
- GitHub Actions CI: lint, typecheck, build, and npm audit on every push/PR
- CodeQL workflow for automated security scanning (runs on push and weekly)

.github/workflows/codeql.yml

@@ -0,0 +1,43 @@
+name: CodeQL Security Analysis
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  schedule:
+    # Run every Monday at 08:00 UTC
+    - cron: '0 8 * * 1'
+
+jobs:
+  analyze:
+    name: Analyze (${{ matrix.language }})
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [javascript-typescript]
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v3
+        with:
+          languages: ${{ matrix.language }}
+          # CodeQL queries to run — security-extended adds more security checks
+          queries: security-extended
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v3
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v3
+        with:
+          category: "/language:${{ matrix.language }}"