Add FastAPI hello-world structure with Docker and DB health check

- app/main.py: root endpoint + /health that reports DB connectivity
- app/db.py: pymysql connection check from MYSQL_* env vars, logged at startup
- Dockerfile: python:3.12-slim, port 8720
- docker-compose.yml: mysql:8.2 with healthcheck, app waits for DB ready
- requirements.txt: fastapi, uvicorn, pymysql

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

Dockerfile

@@ -0,0 +1,10 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY app/ ./app/
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8720", "--log-level", "info"]