ci: add GitHub Actions CodeQL workflow for security scanning

- Create .github/workflows/codeql.yml
- Configure weekly CodeQL analysis on Python code
- Use security-extended and security-and-quality query suites
- Run on push to main, pull requests, and weekly schedule

.github/workflows/codeql.yml

@@ -0,0 +1,41 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+  schedule:
+    - cron: '30 5 * * 2'  # Weekly scan on Tuesdays
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ 'python' ]
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v3
+      with:
+        languages: ${{ matrix.language }}
+        queries: security-extended,security-and-quality
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v3
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v3
+      with:
+        category: "/language:${{matrix.language}}"