refactor: split back into two containers (backend + frontend)

Single-container supervisord approach added unnecessary complexity.
Two containers is simpler and more standard:

- Dockerfile.backend: python:3.11-slim, uvicorn on port 8000
- Dockerfile.frontend: node build → nginx:alpine on port 80
- nginx.conf: proxy_pass restored to http://backend:8000
- docker-compose.yml: two services with depends_on
- Removed combined Dockerfile and supervisord.conf

Each container does one thing; logs are separate; either can be
restarted independently.

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

flight-comparator/Dockerfile.backend

@@ -0,0 +1,30 @@
+FROM python:3.11-slim
+
+ENV PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1 \
+    PIP_NO_CACHE_DIR=1
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY api_server.py airports.py cache.py ./
+COPY database/ ./database/
+
+# Pre-fetch airport data and initialise the database at build time
+RUN mkdir -p data && \
+    python -c "from airports import download_and_build_airport_data; download_and_build_airport_data()" && \
+    python database/init_db.py
+
+EXPOSE 8000
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')" || exit 1
+
+CMD ["python", "api_server.py"]