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/docker-compose.yml

@@ -1,15 +1,28 @@
 services:
-  app:
-    build: .
-    container_name: flight-radar
+  backend:
+    build:
+      context: .
+      dockerfile: Dockerfile.backend
+    container_name: flight-radar-backend
     restart: unless-stopped
     ports:
-      - "80:80"
+      - "8000:8000"
     environment:
       - DATABASE_PATH=/app/data/cache.db
     volumes:
       - flight-radar-data:/app/data
 
+  frontend:
+    build:
+      context: .
+      dockerfile: Dockerfile.frontend
+    container_name: flight-radar-frontend
+    restart: unless-stopped
+    ports:
+      - "80:80"
+    depends_on:
+      - backend
+
 volumes:
   flight-radar-data:
     driver: local