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/dependabot.yml

@@ -0,0 +1,52 @@
+version: 2
+updates:
+  # Frontend npm dependencies
+  - package-ecosystem: "npm"
+    directory: "/frontend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "frontend"
+
+  # Backend npm dependencies
+  - package-ecosystem: "npm"
+    directory: "/backend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "backend"
+
+  # Docker dependencies (docker-compose.yml base images)
+  - package-ecosystem: "docker"
+    directory: "/backend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    labels:
+      - "dependencies"
+      - "docker"
+
+  - package-ecosystem: "docker"
+    directory: "/nginx"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    labels:
+      - "dependencies"
+      - "docker"
+
+  # GitHub Actions workflow dependencies
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    labels:
+      - "dependencies"
+      - "github-actions"

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  frontend:
+    name: Frontend — Lint, Type-check & Build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Type-check
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npm run build
+
+  backend:
+    name: Backend — Lint, Type-check & Build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: backend/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Type-check
+        run: npx tsc --noEmit
+
+      - name: Build
+        run: npm run build
+
+  audit:
+    name: Security Audit
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Audit frontend dependencies
+        run: npm audit --audit-level=high
+        working-directory: frontend
+
+      - name: Audit backend dependencies
+        run: npm audit --audit-level=high
+        working-directory: backend

.github/workflows/codeql.yml

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+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 }}"