Add CI/CD pipeline with Gitea Actions and production deployment

- Production Dockerfiles: backend (gunicorn + uvicorn workers),
  frontend (multi-stage Node build + nginx with API proxy)
- docker-compose.prod.yml: integrates with VPS nginx-proxy via
  VIRTUAL_HOST for auto-TLS at fit.cescalante.dev
- GitHub Actions workflow (Gitea Actions-compatible): builds images
  and deploys on push to main via self-hosted runner
- Make CORS origins configurable via CORS_ORIGINS env var

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Dockerfile.prod

@@ -0,0 +1,16 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt gunicorn
+
+COPY . .
+
+EXPOSE 8000
+
+CMD ["gunicorn", "app.main:app", \
+     "-k", "uvicorn.workers.UvicornWorker", \
+     "-w", "2", \
+     "--bind", "0.0.0.0:8000", \
+     "--timeout", "120"]