Fix postgres port conflict on production server

Use DB_PORT env var (default 5433) to avoid clash with host PostgreSQL on 5432.

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

docker-compose.prod.yml

@@ -46,7 +46,7 @@ services:
   postgres:
     image: postgres:16-alpine
     ports:
-      - "127.0.0.1:5432:5432"
+      - "127.0.0.1:${DB_PORT:-5433}:5432"
     volumes:
       - pg-data:/var/lib/postgresql/data
       - ./db/init.sql:/docker-entrypoint-initdb.d/init.sql