Add GitHub Actions CI (frontend build + backend import/compile)

Closes #6

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ReokyoungKim

.github/workflows/ci.yml

@@ -0,0 +1,44 @@
+name: CI
+
+on:
+  push:
+    branches: [main, dev]
+  pull_request:
+    branches: [main, dev]
+
+jobs:
+  frontend:
+    name: Frontend build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+      - run: npm ci
+      - run: npm run build
+
+  backend:
+    name: Backend import & compile
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    env:
+      # dummy key so module import (which builds no client) is safe in CI
+      OPENAI_API_KEY: sk-ci-dummy
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - run: pip install -r requirements.txt
+      - name: Compile all modules
+        run: python -m compileall -q .
+      - name: Import API app
+        run: python -c "import app; print('app import OK')"