Fix production deployment: replace serve with nginx reverse proxy

Frontend container now uses nginx to serve static files and proxy
/api/* requests to the backend container internally, eliminating
the hardcoded localhost:8000 build-time URL that caused "Network
error" on any non-local server. CORS origins are also configurable
via ALLOWED_ORIGINS env var.

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

btc-portfolio/docker-compose.yml

@@ -8,6 +8,7 @@ services:
     environment:
       - DATABASE_URL=sqlite:////app/data/btc_portfolio.db
       - SECRET_KEY=${SECRET_KEY:-dev-insecure-key-change-me}
+      - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-http://localhost:3000,http://localhost:3001}
     restart: unless-stopped
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8000/')"]
@@ -20,14 +21,14 @@ services:
     build:
       context: ./frontend
       args:
-        - REACT_APP_API_URL=http://localhost:8000
+        - REACT_APP_API_URL=/api
     ports:
-      - "3001:3001"
+      - "3001:80"
     depends_on:
       - backend
     restart: unless-stopped
     healthcheck:
-      test: ["CMD", "wget", "-qO-", "http://localhost:3001/"]
+      test: ["CMD", "wget", "-qO-", "http://localhost:80/"]
       interval: 30s
       timeout: 10s
       retries: 3