ci: add CI workflow to validate frontend build and backend deps

Add a minimal least-privilege CI workflow (contents: read) that validates
the dependency bumps in this branch:
- node: npm ci + lint + next build (typecheck) on Node 20.x/22.x
- python: pip install -r requirements.txt + compileall api scripts (3.11)
- audit: non-blocking npm audit advisory job

Uses actions/checkout@v4, setup-node@v4 (npm cache), setup-python@v5
(pip cache), with a cancel-in-progress concurrency group.

Author: jhamon

.github/workflows/ci.yml

@@ -0,0 +1,63 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  node:
+    name: Frontend (Next.js)
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: ['20.x', '22.x']
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Lint
+        run: npm run lint
+      - name: Build
+        run: npm run build
+
+  python:
+    name: Backend (FastAPI)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+          cache: pip
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+      - name: Byte-compile sources
+        run: python -m compileall api scripts
+
+  audit:
+    name: Dependency audit (advisory)
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22.x'
+          cache: npm
+      - name: npm audit
+        run: npm audit --audit-level=high