chore: add GitHub security hygiene (dependabot, CodeQL, dependency review)

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: weekly
+      day: monday
+    open-pull-requests-limit: 5
+    groups:
+      dev-dependencies:
+        dependency-type: development
+        patterns:
+          - "*"
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+      day: monday
+    open-pull-requests-limit: 3

.github/workflows/codeql.yml

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+name: CodeQL
+on:
+  push:
+    branches: [master, main]
+  pull_request:
+    branches: [master, main]
+  schedule:
+    - cron: "0 6 * * 1"
+permissions:
+  actions: read
+  contents: read
+  security-events: write
+jobs:
+  analyze:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [javascript-typescript]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: github/codeql-action/init@v3
+        with:
+          languages: ${{ matrix.language }}
+      - uses: github/codeql-action/autobuild@v3
+      - uses: github/codeql-action/analyze@v3

.github/workflows/dependency-review.yml

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+name: Dependency review
+on:
+  pull_request:
+    branches: [master, main]
+permissions:
+  contents: read
+  pull-requests: write
+jobs:
+  dependency-review:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/dependency-review-action@v4
+        with:
+          comment-summary-in-pr: always
+          fail-on-severity: high

.github/workflows/security-audit.yml

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+name: Security audit
+on:
+  schedule:
+    - cron: "0 7 * * 1-5"
+  workflow_dispatch:
+permissions:
+  contents: read
+  security-events: write
+jobs:
+  npm-audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+          cache: npm
+      - run: npm ci
+      - name: npm audit (high+)
+        run: npm audit --audit-level=high
+        continue-on-error: true