chore: consolidate to single Docker container

Replace two-container setup (separate backend + nginx frontend) with a
single image that runs both via supervisord:

- New Dockerfile: Node stage builds React, Python+nginx stage is the runtime
- supervisord.conf: manages uvicorn (api_server.py) + nginx as sibling procs
- nginx.conf: proxy_pass updated to localhost:8000 (same container)
- docker-compose.yml: simplified to one service on port 80

Deploy:
  docker-compose up -d        # or
  docker build -t flight-radar . && docker run -p 80:80 flight-radar

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

flight-comparator/nginx.conf

@@ -17,7 +17,7 @@ server {
 
     # API proxy
     location /api/ {
-        proxy_pass http://backend:8000;
+        proxy_pass http://localhost:8000;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';
@@ -30,7 +30,7 @@ server {
 
     # Health check endpoint proxy
     location /health {
-        proxy_pass http://backend:8000;
+        proxy_pass http://localhost:8000;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
     }