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/nginx.conf

@@ -17,7 +17,7 @@ server {
 
     # API proxy
     location /api/ {
-        proxy_pass http://localhost:8000;
+        proxy_pass http://backend: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://localhost:8000;
+        proxy_pass http://backend:8000;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
     }