ci: add CodeQL security scanning and Dependabot

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly
+  - package-ecosystem: npm
+    directory: /frontend
+    schedule:
+      interval: weekly
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/codeql.yml

@@ -0,0 +1,26 @@
+name: CodeQL
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  schedule:
+    - cron: "30 5 * * 1"
+
+jobs:
+  analyze:
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write
+      contents: read
+    strategy:
+      matrix:
+        language: [python, javascript]
+    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