fix: align backend port to 3000 across all configurations

The production docker-compose used port 3000 internally but nginx.conf
was hardcoded to port 3001, causing 502 errors on the root path (/).

Changes:
- Update nginx.conf to use backend:3000
- Update docker-compose.yml to use PORT=3000 for consistency
- Update port mapping and healthcheck to use port 3000

docker-compose.yml

@@ -7,7 +7,7 @@ services:
     restart: unless-stopped
     environment:
       - NODE_ENV=${NODE_ENV:-production}
-      - PORT=3001
+      - PORT=3000
       - JWT_SECRET=${JWT_SECRET}
       - ADMIN_USERNAME=${ADMIN_USERNAME:-admin}
       - ADMIN_EMAIL=${ADMIN_EMAIL:[email protected]}
@@ -43,12 +43,12 @@ services:
       - ./backup:/backup
       - ./storage:/app/storage
     ports:
-      - "${BACKEND_PORT:-3001}:3001"
+      - "${BACKEND_PORT:-3001}:3000"
     depends_on:
       postgres:
         condition: service_healthy
     healthcheck:
-      test: ["CMD", "wget", "--quiet", "--tries=1", "--spider", "http://127.0.0.1:3001/health"]
+      test: ["CMD", "wget", "--quiet", "--tries=1", "--spider", "http://127.0.0.1:3000/health"]
       interval: 30s
       timeout: 10s
       retries: 3