fix: use expose instead of ports for Dokploy/Traefik compatibility

Port 3000 conflicts with Dokploy. Traefik routes traffic via
Docker network, so expose is sufficient. Also remove env_file
refs since Dokploy injects env vars directly.

docker-compose.yml

@@ -4,8 +4,6 @@ services:
       context: ./backend
     expose:
       - "8080"
-    env_file:
-      - .env
     environment:
       - PORT=8080
     healthcheck:
@@ -18,13 +16,13 @@ services:
   frontend:
     build:
       context: ./frontend
-    ports:
-      - "3000:3000"
+    expose:
+      - "3000"
     depends_on:
       backend:
         condition: service_healthy
-    env_file:
-      - .env
+    environment:
+      - API_URL=http://backend:8080
     healthcheck:
       test: ["CMD", "wget", "--spider", "-q", "http://localhost:3000"]
       interval: 30s