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.frontend

@@ -0,0 +1,19 @@
+# ── Stage 1: Build React frontend ─────────────────────────────────────────
+FROM node:20-alpine AS builder
+
+WORKDIR /app
+COPY frontend/package*.json ./
+RUN npm ci
+COPY frontend/ .
+RUN npm run build
+
+# ── Stage 2: Serve with nginx ──────────────────────────────────────────────
+FROM nginx:alpine
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+    CMD wget -q --spider http://localhost/ || exit 1